Fixes usage of WallpaperManager to make it optional.

As the main javadoc states:
"An app can check whether wallpapers are supported for the current user,
by calling {@link #isWallpaperSupported()}, and whether setting of
wallpapers is allowed, by calling {@link #isSetWallpaperAllowed()}."

Test: manual verification on automotive
Test: atest CtsAppTestCases:android.app.cts.WallpaperManagerTest # on automotive

Bug: 138939803

Change-Id: I88946f92ff54e556d289c41c28e22da3a4b8c8b8