Ensure that flow is run on main dispatcher

When adding and removing the callback for KeyguardUpdateMonitor, there
is an assertion that the operation must be on the main thread. The
callback was being removed on a worker thread. We can ensure that this
is not the case with flowOn.

Fixes: 274558013
Test: restart device and unlock the device several times.
Change-Id: I1f40e8b7f2cf7245f0cb17e2d0e10b9ece619234