Workaround missing OnChannelClose for DisplayManagerService

After several doff/don cycles in a stress test, DisplayManagerService
eventually got into a state where it would not allow new connections
despite the old connection being long gone. We confirmed that it
had a display_manager_ pointer set when it should not have, which
means there was a missing OnChannelClose for probably some error
case that directly closed the channel without synthesizing the close.

Those cases should be fixed in followup CLs, but this change will
be helpful to guarantee Display Manager Service continues to operate
regardless. This service is uniquely sensitive to this bug.

Bug: 69004445
Test: Continuously doffed and donned with script
Change-Id: I46c47da38f1024b61dad0f6c19c64aeec53015a6
(cherry picked from commit 8dbbe5584b2fb9589fceed81333a0d5ff6c913a3)