Allow debug hook even if there's no sensor registered

Debug hook is useful even if sensor registration fails for any reason.
Allow brightness override through debug hook even if there's no sensor
registration.

Bug: 116060647
Test: adb shell am broadcast -p com.android.systemui -a com.android.systemui.doze.AOD_BRIGHTNESS --ei brightness_bucket 2
Test: atest DozeScreenBrightnessTest
Change-Id: I49f43af55aa9f9b95255795ac3a332d138cefa61