Expanded wallpaper APIs for keyguard and change detection

* There is a nonzero integer ID associated with the currently-set
  wallpaper image, and this changes every time any caller sets
  the wallpaper.  This is to support external change detection.

* The API now permits a caller to independently set imagery as
  the new general system wallpaper or keyguard-specific wallpaper.
  The backing implementation is not yet plumbed through, but the
  API is now in place for clients to safely write code against.

Bug 25454501
Bug 25454162

Change-Id: I4934f102d5630485bf2870d83664145ae68a3695