Per-user content observer APIs

Callers with INTERACT_ACROSS_USERS_FULL permission can now observe content
for a given user's view (and can notify content uri changes targeted to a
specific user).  An observer watching for UserHandle.USER_ALL will see all
notifications for the given uri across all users; similarly, a notifier
who specifies USER_ALL will broadcast the change to all observers across
all users.

The API handles both USER_ALL or USER_CURRENT, and explicitly forbids
any other "pseudouser" designations.

This CL also revs the Settings provider to notify with USER_ALL for
changes to global settings, and with only the affected user's handle
for all other changes.

Bug 7122169

Change-Id: I94248b11aa91d1beb0a36432e82fe5725bb1264f