Supportleanbackshowcase: Polishing 2 and transition performance fix

Move background of browse/details/music from layout to windowBackground.
Setting background on view will cause the whole RelativeLayout being
faded in while transition move content inside the RelativeLayout.
The RelativeLayout does not have a hardwarelayer, nor does it specify
hasOverlappingRendering=false.  This falls to the slowest rendering
path.

Change-Id: I78bacc965e641d7f960aae5cf212c29f6b1f0f11

samples/SupportLeanbackShowcase/app/src/main/AndroidManifest.xml

@@ -53,7 +53,7 @@
             android:theme="@style/Theme.Example.Leanback"></activity>
         <activity
             android:name=".app.media.MusicExampleActivity"
-            android:theme="@style/Theme.Example.Leanback"></activity>
+            android:theme="@style/Theme.Example.LeanbackMusic"></activity>
     </application>
 
 </manifest>

samples/SupportLeanbackShowcase/app/src/main/res/layout/activity_cards_example.xml

@@ -16,8 +16,6 @@
 -->
 <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
                 android:layout_width="match_parent"
-                android:background="@color/card_examples_background"
-                android:transitionGroup="false"
                 android:layout_height="match_parent">
 
     <fragment

samples/SupportLeanbackShowcase/app/src/main/res/layout/activity_detail_example.xml

@@ -16,8 +16,6 @@
 -->
 <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
                 android:layout_width="match_parent"
-                android:background="@drawable/background_canyon"
-                android:transitionGroup="false"
                 android:layout_height="match_parent">
     <fragment
         android:id="@+id/detailsFragment"

samples/SupportLeanbackShowcase/app/src/main/res/layout/activity_music_example.xml

@@ -16,7 +16,6 @@
 -->
 <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
                 android:layout_width="match_parent"
-                android:background="@drawable/background_sax"
                 android:layout_height="match_parent">
 
     <fragment

samples/SupportLeanbackShowcase/app/src/main/res/values/themes.xml

@@ -27,6 +27,7 @@
     </style>
 
     <style name="Theme.Example.LeanbackBrowse" parent="Theme.Leanback.Browse">
+        <item name="android:windowBackground">@color/card_examples_background</item>
     </style>
 
     <style name="Theme.Example.LeanbackVerticalGrid" parent="Theme.Leanback.VerticalGrid">
@@ -51,5 +52,11 @@
 
     <style name="Theme.Example.LeanbackDetails" parent="Theme.Leanback.Details">
         <item name="android:colorPrimary">@color/detail_view_actionbar_background</item>
-    </style>
+        <item name="android:windowBackground">@drawable/background_canyon</item>
+     </style>
+
+    <style name="Theme.Example.LeanbackMusic" parent="Theme.Example.Leanback">
+        <item name="android:windowBackground">@drawable/background_sax</item>
+     </style>
+
 </resources>