Enable DreamOverlay for Hub on Mobile.

This changelist enables and registers the DreamOverlayService for use
when the Hub on Mobile feature flag is enabled. Eventually, the
AndroidManifest entry for DreamOverlayService should be updated to
reflect the enabled value the supported devices.

Test: atest DreamOverlayRegistrantTest
Flag: com.android.systemui.communal_hub_on_mobile
Bug: 378111815
Change-Id: I07cc89fe70b41b93a4ab893fdb230df2ee4f2d98