Unified screen preview section (2/3).

Implementation of a new section where we show the preview of the lock
screen and home screen and switch between them.

Fix: 263507648
Test: manually verified that switching between the lock screen and home
screen tabs properly updates the preview. Also, scrolling up and down
faithfully moves the preview view on on the screen without rendering
artifacts.

Change-Id: I59a2d70f4a1dee8dda12fdc94a1df03fe5fd4e72