Multi-user ringtone playback.

Change RingtonePlayer to open content:// Uris based on requesting
UserHandle.  Grant SystemUI visibility to all emulated storage so
it can play ringtones for apps without READ_EXTERNAL_STORAGE.

Resolve canonical file:// Uris before passing out of source app,
replacing any /emulated_legacy/-style paths with user-specific
variant so they can be opened by SystemUI.  Calling for RemoteViews,
Ringtones, and Notifications.

Bug: 7202982
Change-Id: Ibf0eca8df80c1486711144a7b648f464aadfe099