UI : Rename 9-patch shadow and add a bottom one

vlc-android/res/layout/shadow_bottom.xml

+<?xml version="1.0" encoding="utf-8"?>
+<View xmlns:android="http://schemas.android.com/apk/res/android"
+    android:layout_width="match_parent"
+    android:layout_height="6dp"
+    android:background="@drawable/shadow_bottom"/>
\ No newline at end of file

vlc-android/res/layout/shadow_top.xml

 <?xml version="1.0" encoding="utf-8"?>
 <View xmlns:android="http://schemas.android.com/apk/res/android"
-    android:id="@+id/shadow"
     android:layout_width="match_parent"
     android:layout_height="6dp"
-    android:background="@drawable/bottom_shadow"/>
\ No newline at end of file
+    android:background="@drawable/shadow_top"/>
\ No newline at end of file