[DO NOT MERGE] Improve brightness handling during screen off timeout.

- Align doze brightness logic with DisplayPowerController, so that the 'minimum dim amount' is taken into account by DozeScreenBrightness.
- Clamp brightness to the now-correct dim value during the entire screen off animation, rather than sometimes transitioning to the sensor value. Apply the sensor brightness value on changing to the DOZE Display state, which happens after the screen off animation.

Bug: 194972547
Test: at 0%, 45%, and 100% brightness, allow the screen to time out in bright and dark rooms, observe minimal brightness changes
Change-Id: I528a944e04f8906cd8c7740eea97dd8fba7c3d3e