Enhance a11y soft keyboard controller

Allow a11y services to request that the soft keyboard be
shown even when the hard keyboard is attached.

Defer to users who override this behavior, and put things
back the way they were when a service requesting this
behavior stops.

Bug: 31012180
Test: Adding CTS tests in linked CL, ran a11y unit tests,
modified TestBack to use the new flag and verified behavior
with a hard keyboard and verfied that settings behave as
expected when overriding and rebooting.
Change-Id: I530481e102ac376a4506b662862ee1ee74815b40