[4/4] Move new picker logic to AOSP

- Move main activity to WallpaperPicker2.

Bug: 190354625
Test: Build and run the app
Change-Id: Ia1ee9df9307345c463d7cd84a641c3654f68877e
Merged-In: Ia1ee9df9307345c463d7cd84a641c3654f68877e
4 files changed