Add settings to control Extended Access mode.

This CL adds settings (and two toggle controls) that help configure
how trust (from Trust Agents) should be interpreted by the platform,
allowing them to function in a purely extend unlock mode (where they
can extend how long an already unlocked device stays unlocked, but
cannot unlock a locked device).

These are temporary settings to help with dogfooding the new behavior,
and will eventually be removed. b/120871688 below is the tracking bug
to remove them.

Bug: 111435975
Bug: 120871688
Test: Tested with SmartLock modalities to confirm behavior in both
legacy and extend unlock modes is WAI.
Change-Id: If25098520ba98e82c98cc51fb226d8f2ce1aba80