AsyncSensorManager: Fix unregistration bug

Fixes an issue where unregistering a listener without specifying a sensor did
not work correctly; when passing null as the sensor, SensorManager.unregisterListener
silently ignores the call; instead, we now call the proper variant if given a null
sensor.

Change-Id: Iac362d8d6ae966308578cb4f177bd37af10b81a2
Fixes: 64357270
Test: runtest -x $T/frameworks/base/packages/SystemUI/tests/src/com/android/systemui/util/AsyncSensorManagerTest.java