Add new preview for theme picker (Part1)

- Uses a flag to hide the new preview until it's ready.
- Uses the apply button of bottom action bar.

Screenshot:
    flag is false: https://screenshot.googleplex.com/nDyDnt708tL.png
    flag is ture: https://screenshot.googleplex.com/R76eoVL0rNy.png

Test: Manually
Bug: 146475648
Change-Id: I49e33e011da106e7773a66a1238cc462a873c1cc