Don't allow dreaming for users that can't dream.

DreamManagerService#canStartDreaming should not return true if the
current user is not allowed to dream.

Bug: 261907079
Test: manually by switching to a user that can't dream and making
sure that the screen turns off on power press while docked.

Change-Id: Ia4a46dff5360d3565762b82fb4353e66f0b8f661