Added gamma correction to autobrightness.

Moved the gamma correction determined by the brightness adjustment to the brightness mapping
strategy, and added a best-effort attempt to gamma correct the curve to meet user data points
before resorting to spline-bending.

Test: manual (see https://drive.google.com/file/d/1VH6LpnmFYTAdRB1Rh_tNmu7xQxEaKlqe/view?usp=sharing
      for instructions).

Fixes: 72734580

Change-Id: I74063f262a5559c048280b6f1d04ac84b0004160