Unregister receiver when destroying TrustAgentWrapper.

Make sure to unregister the receiver when TrustAgentWrapper gets
detroyed. This will prevent over registering for the receiver and a
potential crash if too many recievers are created. The unregistering and
registering of this receiver now take place in the same location as the
other receiver in the class.

Fixes: 221506550
Test: Trivial
Change-Id: Icfd12949d9edb2fa6fbb052db35c16f950328c9a