Add a method in IDreamManager to check if dreaming OR in dream preview

Currently there is no signal to identify that dreaming has started
from the 'preview' option in screen saver settings. This adds a
way to query IDreamManager for this state.

Bubbles needs a way to identify this so that they can hide when
the dream preview starts, otherwise it wouldn't be an accurate
preview.

Bug: 240510360
Test: manual - (with the CL on top of this), have some bubbles and
      trigger dream preview, observe that the bubbles hide.
Change-Id: I5f67a8c793970ada05a010c8d36fd4a47b3f8565