Fix issue #6603152: User-adjustable autobrightness broken?

Two issues:
- The new brightness was not being applied when switching out
  of auto-brightness mode.
- We were resetting to the last brightness level when switching
  modes.  This is because at that point we read the current
  brightness setting, but that isn't changed as the user adjusts
  only when the final value is committed.  To fix this, we have
  a new local variable keeping track of the last brightness value
  we have shown and use that when we haven't committed the
  brightness to settings.

Change-Id: I7cc02c9f90128eacec427587a3f6b8f25ede4bd3