Check for wallpaper support before using APIs

Wallpaper support is not guaranteed and use of WallpaperManager's APIs
need to be guarded behind a WallpaperManager#isWallpaperSupported check
to avoid system service exceptions.

Bug: 355307617
Test: SysUI no longer crashing due to inappropriate access of disabled
wallpaper APIs. When wallpaper support is disabled.
Flag: EXEMPT bugfix

Change-Id: I8fe43c16ff8ece6565951eb423c13e6e4eece1d3