Don't start running FP until FPS is registered

Else, there may be a discrepancy between the
KeyguardUpdateMonitor state and the FPM state if KUM tries to startup
the FPS before it's registered.

Test: atest KeyguardUpdateMonitorTest
Test: manually crash SystemUI when the screen is off,
when turning the screen back on - FPS is running (on devices
with UDFPS, we see the UDFPS affordance)
Fixes: 216718969

Change-Id: I60e75057910f64c59a0c35c46a77763e11540a32