[M3][Color] Set android:windowBackground to android:colorBackground

The same remapping is done on API 21+ by framework base themes. On API levels lower than 21, AppCompat base themes "backport" the behavior to map window background to a separate color resources. With Material 3 we want a consistent and clear behavior, this CL explicitly remaps android:windowBackground to android:colorBackground so the behavior will be the same across all API levels.

PiperOrigin-RevId: 417711790

lib/java/com/google/android/material/theme/res/values/themes_base.xml

@@ -51,6 +51,9 @@
     <item name="colorErrorContainer">@color/m3_sys_color_light_error_container</item>
     <item name="colorOnErrorContainer">@color/m3_sys_color_light_on_error_container</item>
 
+    <!-- Framework Colors. -->
+    <item name="android:windowBackground">?android:attr/colorBackground</item>
+
     <!-- Default Framework Text Colors. -->
     <item name="android:textColorPrimary">@color/m3_default_color_primary_text</item>
     <item name="android:textColorPrimaryInverse">@color/m3_dark_default_color_primary_text</item>
@@ -245,6 +248,9 @@
     <item name="colorErrorContainer">@color/m3_sys_color_dark_error_container</item>
     <item name="colorOnErrorContainer">@color/m3_sys_color_dark_on_error_container</item>
 
+    <!-- Framework Colors. -->
+    <item name="android:windowBackground">?android:attr/colorBackground</item>
+
     <!-- Default Framework Text Colors. -->
     <item name="android:textColorPrimary">@color/m3_dark_default_color_primary_text</item>
     <item name="android:textColorPrimaryInverse">@color/m3_default_color_primary_text</item>