Floss: Auto reply Just Works pairing

This makes Floss auto reply ssp request if it is Just-work pairing.
For incoming pairing, auto reject it. For outgoing pairing, auto accept
it.

This doesn't really change the behavior since we also reject/accept in
the chrome anyway.

Bug: 304925828
Test: tested with another chromebooks and verify both incoming and
outgoing works as expected. Also check dbus-monitor to verify there is
no SspRequest callback.
Test: mma -j
Tag: #floss

Change-Id: Ib3f826727f59e449d6c45b12a153388395391a2f