Don't report slider events with bad lux readings.

Wait until AutomaticBrightnessController has a valid
lux reading before reporting slider events.

Bug: 126683965
Test: atest android.display.cts.BrightnessTest with config_lightSensorWarmupTime set to 0 - test passes
Test: atest android.display.cts.BrightnessTest with config_lightSensorWarmupTime set to 3000 - test passes
Test: atest android.display.cts.BrightnessTest with config_lightSensorWarmupTime set to 4000 - test times out
Change-Id: I881427c85835e751ba5ac75086a9612984878e13