Stay registered to proximity sensor while activities are occluding keyguard

When a phone call is received while the device's prox sensor is covered (such as when it's face down on a table or in a pocket), we run falsing on all gestures while in the activity as if the sensor is still covered, because we unregister from further updates from the prox sensor once StatusBarState changes from KEYGUARD. This change keeps us subscribed to the prox sensor so we aren't left with stale state in this scenario.

Bug: 297497511
Test: added a unit test, passes existing tests for sensor
Test: manually checked we still unregister from proximity sensor updates when unlocked, in AOD, in lockscreen shade/QS, in bouncer
Flag: NONE
Change-Id: I55f1472593d37f47574d4bf3c40ca7d69f86bad8