Manage wallpaper connections on display changes

There are three different type of wallpapers
1. Fallback wallpaper: used when the user selected wallpaper doesn't support the added display.
2. System wallpaper: a user selected wallpaper or default. Used in external displays when supported.
3. Lock wallpaper: if the user selects a wallpaper specifically for the lock screen.

Also, make the following changes in SysUi to show wallpaper in connected display keyguard:
1. Make the background of ConnectedDisplayKeyguardPresentation transparent.
2. Add FLAG_SHOW_WALLPAPER to ConnectedDisplayKeyguardPresentation's window

TODO:
1. The logic for determining if a wallpaper supports an added display will be added in a subsequent CL.
2. The updateFallbackConnection will be updated to handle multi displays in a subsequent CL.

Test: atests FrameworksMockingServicesTests:WallpaperManagerServiceTests. Also, connect to an external display to test wallpapers are shown correctly in home screen and keyguard.
Flag: android.app.enable_connected_displays_wallpaper
Bug: 384520326
Change-Id: Ieca76e8e67af8f12afc145b3dc6050771e3419d3