Add a comment in WallpaperManager#getWallpaperColors

After discussion, instead of preventing the user get stuck
when calling WallpaperManager#getWallpaperColors, we decided
to inform developers that this API should not be called in
their UI thread.

Bug: 127343255
Test: atest WallpaperManagerTest
Change-Id: I3ae82a127391d4d8c23b56077954b74fd691f774
1 file changed