Hook up AOD knobs to Settings.Global

This cl creates AlwaysOnDisplayPolicy, which get values for the
following knobs(go/aod-experiments) from Settings.Global:
1. screen_brightness_array
2. dimming_scrim_array
3. prox_screen_off_delay
4. prox_cooldown_trigger
5. prox_cooldown_period

Also update code to make sure AlwaysOnDisplayPolicy is used
everywhere.

Bug: 64899561
Test: runtest -x AlwaysOnDisplayPolicyTest

Change-Id: I2e83ff980771e67177e4964bd83aa68b2bdca65f