Dismiss stage split when unlock after folded

Dismiss stage split to the last focused stage when unlocking after
folded.

Bug: 184795546
Test: enter split and fold the device, observe it dismisses to the
      proper stage after unlocked the device.
Test: enter split after folded, unlock whon't dismiss split.

Change-Id: I6abbf63c1675d612811c47b1c42f337607fb65fd