Add hidden system dream API in DreamManagerService.

This change
- adds a new private API in dream manager that allows a client with
  write dream permission to set a system dream component, which takes
  precedence over user configured dream
- fixes startDream() API in DreamManager which shouldn't take in a
  component name

Test: atest SystemDreamTest
Bug: 222529147
Change-Id: Iee9a0af1071e5ed57fa5bd4b0d06d8178118116e