Refactor Dream Overlay.

This changelist refactors dream overlay to distinguish
the overlay from the configured components it houses.
These components are hereafter referred to as
complications.

Test: atest ComplicationPrimerTest
Test: atest ComplicationProviderTest
Test: atest DreamOverlayServiceTest
Test: atest DreamOverlayStateControllerTest
Bug: 211446330
Change-Id: I311bf8f1d7977e74bbd96b6ddf2419b8f0613626