Extract QSHost interface

In preparation for refactor, extract the QSHost interface everywhere so
access is done through it. This determines also the surface we may need
to provide.

Test: atest SystemUITests
Test: atest android.host.systemui
Test: atest CtsTileServiceTestCases
Bug: 249804331
Change-Id: I660311eb60336ead6e5694c4e3e6099151dbaecc