Make ZenModeRepository.modes a (single) StateFlow

Using regular Flows per caller means that every client of ZenModeRepository/ZenModeInteractor causes a new ContentObserver to be registered (and ZenModesBackend.getModes() to be called on each of those changes). With the current clients of the interactor, this could easily become 12 or more Binder calls per each zen change!

Bug: 403434908
Test: atest ZenModeRepositoryTest + manual
Flag: EXEMPT Simple (and urgent) performance improvement
Change-Id: I428ef6ef03eb331115d53f878e742542963a11a8