Add test only methods to enable fallback wallpaper testing

This is a stop gap solution to enable fallback wallpaper testing before we add display resolution & image wallpaper aspect ratio check.

Test: atests FrameworksMockingServicesTests:WallpaperManagerServiceTests
Flag: android.app.enable_connected_displays_wallpaper
Bug: 384520326
Change-Id: I373940ef48eddf1726d55158ff43b06b384344ce