power: Delete disable_als pref.

This isn't used anymore. The has_ambient_light_sensor pref
can be set to 0 to make powerd avoid using the ALS.

Also update AmbientLightHandler to assert at initialization
that its prefs are sane.

BUG=chromium:356395
TEST=manual: ambient light sensor still works on pixel
CQ-DEPEND=Idb5da0ea393468933f8d7d2ca6c845e2afa1c433

Change-Id: I760ee019bdfd0b439a66e6cfec87d8ccb97965f1
Reviewed-on: https://chromium-review.googlesource.com/191824
Tested-by: Daniel Erat <derat@chromium.org>
Reviewed-by: Chris Masone <cmasone@chromium.org>
Commit-Queue: Daniel Erat <derat@chromium.org>
9 files changed