Change the default behavior with status bar decouple flag

The default behavior will be the devices default behavior, and the flag
will be a hint to enable the override to provide a new behavior override
beforehand.

Also, prepare the override types as a full decouple as it makes no more
sense if we don't apply it by default to decouple only parts of the
system bars.

Rename the legacy thing to override, as the override can happen in two
directions.

Test: DisplayPolicyInsetsTests
Test: DisplayPolicyTests
Bug: 327313645
Change-Id: I6f9de120bd29bfeb9f02ddf2c36fbe44f512a1b8