Add Unit test for WallpaperManagerService.

Add some basic tests based on previous bugs we met in Q.

- Launched wallpaper components shall be expected.
- Display size used to generate crop image must be sane size.
- No connect left after switch user continuously.
- Display size and launched component should be expected after switch
user and unlock device.

Bug: 129797242
Test: atest FrameworksMockingServicesTests:WallpaperManagerServiceTests
atest WallpaperManagerTest
Change-Id: Ifae2c5f4868cedb26e27a3d76fe5d61002208536