Make wallpapers direct-boot aware.

If the user's wallpaper isn't direct-boot aware, wait around for
the user to be unlocked, instead of clearing the wallpaper.

Also switch a few classes to using SystemService lifecycle, since
events are dispatched faster than through broadcasts.  Fix bug where
ContentService.systemReady() was never called, and make sure
EntropyMixer doesn't risk being GC'ed.

Bug: 26280055
Change-Id: I9fff468a439b868baa68cf11bb6ee9f7d52b7b5a