Lock if device unplugged and sleeping

If external display is unplugged, and leaves only sleeping displays
remaining, lock the device.

Bug: 414558760
Flag: com.android.server.power.feature.flags.lock_on_unplug
Test: atest PowerManagerServiceTest
Change-Id: Ic4dc7f01a6ebbe4509cb2a50f14a27bc7f27c46b