Fix fallback wallpaper connections after reboot

Issues:
1. After a reboot, fallback wallpaper connections are not established correctly because the DisplayData is created only after a display is connected.
2. Test issues:
    2.1: wrong assumption: tests previously added for fallback wallpaper connection tests for multi-displays are skipped. Remove these assumptions in this change.
    2.2: getConnectedEngineSize only returns the size of the display connectors that have a non-null engine. We don't stub the engine during these tests. Replaced this assertion with display connector display id assertions.
    2.3: added a TestWallpaperService to stimulate user changing wallpaper.

Test: atests FrameworksMockingServicesTests:WallpaperManagerServiceTests
Flag: android.app.enable_connected_displays_wallpaper
Bug: 384520326
Change-Id: Ic1368aa5d2045d5cf3ff5057e428ae4f7791df3e