Migrate WallpaperActivity to FragmentActivity

This CL introduces few improvements to DesktopWallpaperActivity:

- Instead of base activity class, use FragmentActivity. This way we can
use more up to date `onBackPressedDispatcher` to handle back navigation
on the activity.
- Ensure that the back navigaiton moves the task back rather
than finishing the activity.
- Add multivalent unit tests for the activity.

Test: atest WMShellMultivalentTestsOnDevice:DesktopWallpaperActivityTest
Bug: 419286896
Flag: com.android.window.flags.enable_empty_desk_on_minimize
Change-Id: Icf9e6f485e04c4d9309fe1b21ff03f35724a9723