Tabbed navigation in Wallpaper Picker (2/3).

Introduces tabbed navigation to switch between home and lock screen in
wallpaper picker.

Fix: 262924054
Test: Includes new unit tests for new view-model
Test: Manually verified that switching back and forth keeps the previews
alive and selectively switches the bottom few sections in a nice,
animated way with minimal/no jank.

Change-Id: Iebc3f9e0ccdf8296e90125092197500c1fee7c4f