Add short term model reset threshold

Add a threshold to prevent user birghtness adjustments from reseting
without a drastic enough change in the ambient light, and some minor
refactoring

Test: manually adjust brightness; turn screen off for 30 seconds;
      turn screen back on; brightness shouldn't change
Test: manually adjust brightness; turn screen off for 30 seconds;
      cover light sensor; turn screen back on; brightness should change

Make short term model reset decision only happen once, and change its
threshold to be relative

Test: same as before

Minor refactoring

Test: same as before
Fixes: 72734580

Change-Id: I724e88ddb79a55cadb547463d73131028bb57825