Add plugin hooks to DozeService

- Find out when the dreaming starts / stops
- Request the doze UI to show / hide

Bug: 62534409
Test: manual, have no plugin, check that doze still works as expected (i.e.
      pick up gesture shows it, noti shows it)
Change-Id: I11618e47e0bab18423f0d89c8daf285d8c14d238