Add Settings keys for Wear Autobrightness Values

This has been created/used in Wear (ag/18494126), but since the CL that
adds that has more logic changes in the display code space, we're
creating a new CL here specifically for creating the Settings keys.

Bug: 303443082
Test: unit tests
Change-Id: I234590fc1b0aedfa0cf6454697f41015ec49a4ed