Added missing lock and clean up locked function that wasn't locked

There was a missing lock that systemui would call to register a docked
stack listener. Since it was missing, it is assumed that two threads
went into a race condition which caused it to crash when handling begin
before finish broadcasts for the same DockedStackDividerController.
Also moved getDefaultDisplayContentLocked call that was not locked to a
locked area.

Change-Id: I3c7ef805024aaf0f5f7c4ab68fc779797114a038
Fixes: 62104210
Test: go/wm-smoke