On registerForDismissGestures coroutine completion, unregister for dismiss gestures

Else, the touch handlers will continue to be registered when they're
undesirable.

Flag: com.android.systemui.device_entry_udfps_refactor
Fixes: 341217019
Test: Enroll fingerprint on device that supports the alternate bouncer.
Double tap the power button to trigger the camera activity. Press an
affordance (gallery) to trigger the alternate bouncer. Tap on the screen
to get the primary bouncer. Navigate back gesture to the camera
activity. Observe that camera activity still works normally. Touches
are not intercepted.

Change-Id: I2bf9452009b2782fda343631b7fdb8c493758fa1