AOD: Prewarm display while waiting for brightness sensor

This partially reverts commit a79ad59d.

Instead of keeping AOD paused, we now turn the display on, but keep it soft
blanked by keeping the front scrim opaque black until we get a valid sensor
event.

Bug: 65010918
Test: runtest -x packages/SystemUI/tests/src/com/android/systemui/doze/DozeScreenBrightnessTest.java
Change-Id: Ic39d29814024d52117538a6a19adaf3866c7223e