Content observer callbacks don't specify the user,
so scan all the users we know about. User switches
will also trigger an update which will explicitly
check the setting for the (now) current user.
Migrate tests to TestableContext to allow the test
to safely override secure settings.
Bug: 36021111
Test: runtest systemui-notification
Change-Id: I588068382912955da114cc351df7a1e7b4f0733f
89aa226190