Fixed some android build errors

(cherry picked from commit d623e24c)

# Conflicts:
#	src/LibVLCSharp.Forms/Platforms/Android/SystemUI.cs
#	src/LibVLCSharp.Forms/Platforms/Android/VideoViewRenderer.cs

samples/Forms/LibVLCSharp.Forms.MediaElement/LibVLCSharp.Forms.MediaElement.Sample.Android/Resources/Resource.designer.cs

@@ -2,7 +2,6 @@
 //------------------------------------------------------------------------------
 // <auto-generated>
 //     This code was generated by a tool.
-//     Runtime Version:4.0.30319.42000
 //
 //     Changes to this file may cause incorrect behavior and will be lost if
 //     the code is regenerated.

samples/Forms/LibVLCSharp.Forms.Sample.Android/Resources/Resource.designer.cs

@@ -2,7 +2,6 @@
 //------------------------------------------------------------------------------
 // <auto-generated>
 //     This code was generated by a tool.
-//     Runtime Version:4.0.30319.42000
 //
 //     Changes to this file may cause incorrect behavior and will be lost if
 //     the code is regenerated.

samples/LibVLCSharp.Android.Sample/Resources/Resource.Designer.cs

@@ -2,7 +2,6 @@
 //------------------------------------------------------------------------------
 // <auto-generated>
 //     This code was generated by a tool.
-//     Runtime Version:4.0.30319.42000
 //
 //     Changes to this file may cause incorrect behavior and will be lost if
 //     the code is regenerated.

src/LibVLCSharp.Forms/Platforms/Android/Platform.cs

@@ -23,34 +23,34 @@ namespace LibVLCSharp.Forms.Platforms.Android
                 set => CurrentActivity.SetTarget(value);
             }
 
-            public void OnActivityCreated(Activity activity, Bundle savedInstanceState)
+            public void OnActivityCreated(Activity? activity, Bundle? savedInstanceState)
             {
                 Activity = activity;
             }
 
-            public void OnActivityDestroyed(Activity activity)
+            public void OnActivityDestroyed(Activity? activity)
             {
             }
 
-            public void OnActivityPaused(Activity activity)
+            public void OnActivityPaused(Activity? activity)
             {
                 Activity = activity;
             }
 
-            public void OnActivityResumed(Activity activity)
+            public void OnActivityResumed(Activity? activity)
             {
                 Activity = activity;
             }
 
-            public void OnActivitySaveInstanceState(Activity activity, Bundle outState)
+            public void OnActivitySaveInstanceState(Activity? activity, Bundle? outState)
             {
             }
 
-            public void OnActivityStarted(Activity activity)
+            public void OnActivityStarted(Activity? activity)
             {
             }
 
-            public void OnActivityStopped(Activity activity)
+            public void OnActivityStopped(Activity? activity)
             {
             }
         }
@@ -73,7 +73,13 @@ namespace LibVLCSharp.Forms.Platforms.Android
             {
                 lifecycleListener = new ActivityLifecycleContextListener();
                 LifecycleListener = lifecycleListener;
-                activity.Application.RegisterActivityLifecycleCallbacks(lifecycleListener);
+                var app = activity.Application;
+                if (app is null)
+                {
+                    throw new InvalidOperationException("The given activity is not linked to an Application instance (activity.Application is null)");
+                }
+
+                app.RegisterActivityLifecycleCallbacks(lifecycleListener);
             }
             lifecycleListener.Activity = activity;
         }

src/LibVLCSharp.Forms/Platforms/Android/SystemUI.cs

-using Android.App;
-using Android.Views;
-using Java.Lang;
+using Android.Views;
 using LibVLCSharp.Forms.Platforms.Android;
 using LibVLCSharp.Forms.Shared;
 using Xamarin.Forms;
@@ -12,21 +10,23 @@ namespace LibVLCSharp.Forms.Platforms.Android
     {
         public void ShowSystemUI()
         {
-            if (Platform.Activity == null)
+            var decorView = Platform.Activity?.Window?.DecorView;
+            if (decorView == null)
                 return;
 
-            Platform.Activity.Window.DecorView.SystemUiVisibility =
+            decorView.SystemUiVisibility =
                 (StatusBarVisibility)(SystemUiFlags.LayoutStable |
                 SystemUiFlags.LayoutFullscreen);
         }
 
         public void HideSystemUI()
         {
-            if (Platform.Activity == null)
+            var decorView = Platform.Activity?.Window?.DecorView;
+            if (decorView == null)
                 return;
 
-            Platform.Activity.Window.DecorView.SystemUiVisibility = 
-               Platform.Activity.Window.DecorView.SystemUiVisibility |
+            decorView.SystemUiVisibility = 
+               decorView.SystemUiVisibility |
                (StatusBarVisibility)(SystemUiFlags.ImmersiveSticky | 
                SystemUiFlags.Fullscreen | 
                SystemUiFlags.HideNavigation |

src/LibVLCSharp.Forms/Platforms/Android/VideoViewRenderer.cs

@@ -35,7 +35,7 @@ namespace LibVLCSharp.Forms.Platforms.Android
             {
                 if (Control == null)
                 {
-                    SetNativeControl(new LibVLCSharp.Platforms.Android.VideoView(Context));
+                    SetNativeControl(new LibVLCSharp.Platforms.Android.VideoView(Context!));
 
                     e.NewElement.MediaPlayerChanging += OnMediaPlayerChanging;
                     if (Control!.MediaPlayer != e.NewElement.MediaPlayer)