Add scroll indicators to GLIF Pixel theme

Add the scroll indicators to clearly indicate the divide between
status bar, content area, and footer, even though they all have the
same background color.

The scroll indicator will only be shown if there are content beyond
the fold.

Test: ./gradlew connectedAndroidTest test
Bug: 35446596
Change-Id: I5419e4f443a2414f8c983241b8fc61cadb48ef68