Update top of modes page

* Show blurb based on mode type.
* Make the icon bigger and highlight it when mode is active.
* Increase spacing between elements.

Also eliminate some code duplication between header of mode page and header of icon picker.

Fixes: 355415875
Test: manual
Flag: android.app.modes_ui
Change-Id: I7e788b9b5920cedb791d1571b19b37e65ece6d0b

res/layout/modes_activation_button.xml

@@ -18,7 +18,8 @@
 <FrameLayout
     xmlns:android="http://schemas.android.com/apk/res/android"
     android:layout_width="match_parent"
-    android:layout_height="wrap_content">
+    android:layout_height="wrap_content"
+    android:paddingVertical="8dp">
 
     <Button
         android:id="@+id/activate_mode"