Make some apis @TestApi and @hide for usage in tests

Some apis in class, WallpaperManager and Display, are not visible in
tests, make them visible for tests usage.

Bug: 142845271
Test: atest android.display.cts.DisplayTest#testGetSupportedWideColorGamut
--rerun-until-failure 10
Test: atest android.app.cts.WallpaperManagerTest#testWallpaperSupportsWcg
--rerun-until-failure 20
Test: atest android.app.cts.WallpaperManagerTest#testWideGamutWallpaper
--rerun-until-failure 20

Change-Id: I1958df1e53aa7f65a2eb9ee7fe2a6b098ac2ab39