Per-user settings

Each user has its own Settings.System.* and Settings.Secure.* namespace now.  In
addition, this CL introduces the new Settings.Global.* namespace, which contains
a number of previously-elsewhere named settings entities; these Global.* entities
are common to all users.  Because these elements have been moved from their prior
existence in the other namespaces, attempts to access them under their old names
and namespaces are detected and redirected (with appropriate compile-time and
logging messages) to their new homes.

The new Global.* namespace can only be written by system-level code, just like
the existing Secure.* namespace.  If an app attempts to write a key that was
previously in the System.* namespace but has been moved to the Global.* namespace,
then a warning is logged and no write is performed; the action is a no-op.  (The
app is explicitly not crashed, to avoid breaking well-behaved apps that can't
know any better.)

There is also now a hidden API for getting/setting settings entities associated
with a user other than the caller's.  Reading/writing data for a user other than
yourself requires the signature-level INTERACT_ACROSS_USERS_FULL permission.

Manipulating data for a different user cannot be done via the ContentProvider
query() / insert() APIs; you must use the Settings.get/put APIs for that degree
of control.  In general, use of the get/set API is *strongly* preferred over
query-type access to Settings.

Bug 6985398

Change-Id: Ibee54ddff99fb847c8c2479c23b50f1e7524d724