Camera: Don't hold 'mLock' when closing the camera session

After flushing the camera requests don't try to close the
camera session while still holding 'mLock'. Different Hals
could still try and return additional results which then will
get blocked on 'mLock'. Closing the session could potentially
introduce a deadlock if the Hal implementation waits for any
of the result calls to return.

Bug: 111836165
Test: vts-tradefed run commandAndExit vts --skip-all-system-status-check
--skip-preconditions --module VtsHalCameraProviderV2_4Target -l INFO

Change-Id: I0f309f435cc76db3f9b6ba500da65a232b014717