Backup OS service now processes unlock off the main thread

We now perform the slow work of unlocking the primary user on the
background handler thread rather than on the main thread.  This
is expected to shave 150ms+ off of our total unlock latency on
some products.

Bug: 63389800
Test: manual
Change-Id: I70be032a64de3270476615fd9dde1b427e19aa91