Restrict reflection access to Toast's TransientNotification.

Apps accessing the transient notification end up modifying the Toast's
presentation in un-intended ways, such as forcing it to show on top of
the lock screen. For other cases, as configuration of duration, gravity
or view to show, Toast has available public APIs.

Bug: 123768598
Test: n/a
Change-Id: I0854389e4cc60baff2d433d4c49fa603a242982e