Add isDreaming method to DreamManager

This CL adds isDreaming to DreamMaanger and changes the way it starts a
dream to use IDreamManager.dream()

DreamManager is only used for testing. So far it has been using the
DreamManagerInternal.testDream() API to start the dream. This restricts
the amount of verification that can be done in the dream tests because
it doesn't put the device in a dreaming state -
IDreamManager.isDreaming() is always false.
IDreamManager.dream() puts the device in a dreaming state and enables
better testing.

Bug: 152994058

Test: atest DreamManagerServiceTests
Change-Id: Id4d947e83eabcafa9724764b8d063357c5f2cb49