5/N Reduce number of accessors of root component.

Ensure that only the code that actually needs the root component asks
for it.

This also fixes ContentProvider injection (a bit). They no longer have
to manually run injection on themselves, as long as they implement
SystemUIAppComponentFactory.ContextInitializer properly and define
an inject(MyClass) method in the root component.

Bug: 162923491
Test: manual && atest SystemUITests
Change-Id: I507859f5f8d89bdde5b10e78b54c5cfbf28737bc