Notify TrustListeners when enabled trust agents change.

This change adds a new callback from TrustManagerService when the
enabled trust agents change. This addition enables TrustListeners to
react when an authentication method has been enabled by the user.

Test: atest KeyguardStateControllerTest#testOnEnabledTrustAgentsChangedCallback
Test: atest KeyguardUpdateMonitorTest#testOnEnabledTrustAgentsChangedCallback
Test: atest TrustManagerServiceTest#reportEnabledTrustAgentsChangedInformsListener
Fixes: 277845892
Fixes: 279231562
Change-Id: Id6d4b65abb4de77f52f1d48499eed3ca26384663