[Pod] Add interface for SystemUIApplication.

This will let SystemUIApplicationImpl live in our SystemUI-application
build target, while services in SystemUI-core can still reference
SystemUIApplication.

Bug: 307607958
Flag: EXEMPT refactor
Test: switch between users -> verify each user operates as before
Test: Verify recents button in secondary user still works
Test: atest SystemUIApplicationImplTest
Change-Id: I5d5c6c38bfa1832b21e83af40f83659584149147