Let external activities run when the primary display is off

- Make activity sleep state independent of power manager wakefulness
  state. The state is now entirely dependent on sleep tokens (and the
  voice interactor).
- Make sleep tokens operate on a per-display basis (and convert the
  keyguard to a sleep token).
- Make ActivityStackSupervisor acquire/release sleep tokens for
  non-default displays when the displays are turned on/off.
- Make WindowManagerService.okToDisplay operate on a per-display basis.

Bug: 34280365
Test: android.server.cts.ActivityManagerDisplayTests
Test: #testExternalDisplayActivityTurnPrimaryOff
Test: #testLaunchExternalDisplayActivityWhilePrimaryOff
Test: #testExternalDisplayToggleState
Change-Id: I92086d7006a67b4b4f320c9bb3aa606954f85012