Multiuser awareness in content observer infrastructure

Content observers are registered under the calling user's identity,
not under the provider host's identity (unless the caller is using
the new permissioned entry points that allow observers to be
registered for a different user's view of the data).  The most important
implication of this is that when the settings provider is directly
queried, the Cursor returned to the app is wired for change notifications
based on that calling app's user.

Note that it is not possible to use query() / insert() to read/write
data for different users.  All such manipulations should use the
standard get* / put* methods in Settings.*.

Bug 7122169

Change-Id: If5d9ec44927e5e56e4e7635438f4ef48a5430986