Fixed auto-brightness when ALS malfunctions.

When the ALS is broken or misbehaving, auto-brightness might get
stuck in a state where it completely ignores user data points;
this fixes the issue, and reverts to manual brightness whenever
auto-brightness can't be used reliably.

Test: manual.

Change-Id: I5eb80b99dbf7cc25229dc217732515b2a4ac1b79
Fixes: 77280930