Make ZenIconLoader injectable (in SystemUI)

And provide a direct-executor-service version for ZenModeInteractor in Kosmos.

(This allows us to get rid of all usages of ZenIconLoader.setInstance(), which was used exclusively for this exact purpose).

Bug: 360399800
Test: atest ModesTileTest ZenModeInteractorTest
Flag: android.app.modes_ui
Change-Id: I16219ef486b17b2e5f6819008bcc6fb0697b0f80