Add setManagingTrust and expose it on lockscreen

Adds a facility for trust agents to indicate if they
are ready to manage trust. Also adds an indication to
the lock icon on the lockscreen to show whether trust is
being managed.

Bug: 15518469
Bug: 16123013

Change-Id: Ie17f588aebeafe66c81dea4a69c733b0d2c72fd4