Cleanup keyguard state controller callback.

ScrimManager only needs to track the keyguard state while there is a
registered callback. This change moves the registration to be scoped to
when listeners are present.

Fixes: 428673574
Test: atest ScrimManagerTest#testKeyguardStateCallbackRegistration
Flag: EXEMPT bugfix
Change-Id: I19a249fb2f74e5b3cd747d3fb7ffd4e8a02054ef