[flexiglass] Reset PIN and Password on the bouncer when it's re-shown.

Fix: 291999047
Test: Added unit tests, existing ones still pass.
Test: Manually tested on a device following the repo steps on the
associated bug, observed that the bug doesn't reproduce due to the fix.

Change-Id: I285d15d862c3b7cacd1432bf20278a292ff4cd8d