Fix RTL problems onboarding next and prev buttons

Fixes #1578 problems 1,2,3

application/resources/src/main/res/drawable/ic_onboarding_next.xml

+<vector android:height="48dp" android:tint="#FFFFFF"
+    android:autoMirrored="true"
+    android:viewportHeight="24" android:viewportWidth="24"
+    android:width="48dp" xmlns:android="http://schemas.android.com/apk/res/android">
+    <path android:fillColor="@android:color/white" android:pathData="M10,6L8.59,7.41 13.17,12l-4.58,4.59L10,18l6,-6z"/>
+</vector>

application/resources/src/main/res/drawable/ic_onboarding_next_pressed.xml

+<vector android:height="48dp" android:tint="#FFB54C"
+    android:autoMirrored="true"
+    android:viewportHeight="24" android:viewportWidth="24"
+    android:width="48dp" xmlns:android="http://schemas.android.com/apk/res/android">
+    <path android:fillColor="@android:color/white" android:pathData="M10,6L8.59,7.41 13.17,12l-4.58,4.59L10,18l6,-6z"/>
+</vector>

application/resources/src/main/res/drawable/ic_onboarding_previous.xml

+<vector android:autoMirrored="true" android:height="48dp"
+    android:tint="#FFFFFF" android:viewportHeight="24"
+    android:viewportWidth="24" android:width="48dp" xmlns:android="http://schemas.android.com/apk/res/android">
+    <path android:fillColor="@android:color/white" android:pathData="M15.41,7.41L14,6l-6,6 6,6 1.41,-1.41L10.83,12z"/>
+</vector>

application/resources/src/main/res/drawable/ic_onboarding_previous_pressed.xml

+<vector android:autoMirrored="true" android:height="48dp"
+    android:tint="#FFB54C" android:viewportHeight="24"
+    android:viewportWidth="24" android:width="48dp" xmlns:android="http://schemas.android.com/apk/res/android">
+    <path android:fillColor="@android:color/white" android:pathData="M15.41,7.41L14,6l-6,6 6,6 1.41,-1.41L10.83,12z"/>
+</vector>

application/resources/src/main/res/drawable/ic_widget_onboarding_next_w.xml

 <?xml version="1.0" encoding="UTF-8"?>
-<selector xmlns:android="http://schemas.android.com/apk/res/android">
+<selector xmlns:android="http://schemas.android.com/apk/res/android" android:autoMirrored="true">
     <item android:drawable="@drawable/ic_onboarding_next" android:state_focused="true" android:state_pressed="true" />
     <item android:drawable="@drawable/ic_onboarding_next_pressed" android:state_focused="true" />
     <item android:drawable="@drawable/ic_onboarding_next_pressed" android:state_pressed="true" />

application/resources/src/main/res/drawable/ic_widget_onboarding_previous_w.xml

 <?xml version="1.0" encoding="UTF-8"?>
-<selector xmlns:android="http://schemas.android.com/apk/res/android">
+<selector xmlns:android="http://schemas.android.com/apk/res/android" android:autoMirrored="true">
     <item android:drawable="@drawable/ic_onboarding_previous" android:state_focused="true" android:state_pressed="true" />
     <item android:drawable="@drawable/ic_onboarding_previous_pressed" android:state_focused="true" />
     <item android:drawable="@drawable/ic_onboarding_previous_pressed" android:state_pressed="true" />

application/vlc-android/res/layout/activity_onboarding.xml

@@ -21,8 +21,8 @@
             android:id="@+id/previous"
             android:layout_width="wrap_content"
             android:layout_height="wrap_content"
-            android:layout_marginStart="16dp"
-            android:layout_marginBottom="16dp"
+            android:layout_marginStart="8dp"
+            android:layout_marginBottom="8dp"
             android:background="@color/transparent"
             app:layout_constraintBottom_toBottomOf="parent"
             app:layout_constraintStart_toStartOf="parent"
@@ -86,8 +86,8 @@
             android:id="@+id/next"
             android:layout_width="wrap_content"
             android:layout_height="wrap_content"
-            android:layout_marginEnd="16dp"
-            android:layout_marginBottom="16dp"
+            android:layout_marginEnd="8dp"
+            android:layout_marginBottom="8dp"
             android:background="@color/transparent"
             app:layout_constraintBottom_toBottomOf="parent"
             app:layout_constraintEnd_toEndOf="parent"