Allow acquiring ContentProviders across users.

Otherwise services like SystemUI will always open content://-style
Uris as USER_OWNER.  Surfaces through createPackageContextAsUser()
which points all ContentResolver operations towards a given user.

Start using in RemoteViews, so that Notifications correctly resolve
image Uris to the sending user.  Also add user support for "content"
shell tool.

Bug: 7202982
Change-Id: I8cb7fb8a812e825bb0b5833799dba87055ff8699