Change SysemUIFactory to SystemUIInitializer.

This change allows variants of SystemUI to remove the reflection
traditionally used to startup systemui and instead directly stand up
the code that they need.

Specifically, it introduces two primary pieces:

- SystemUIInitializer
  A rename of the SystemUIFactory along with some modest refactoring.
  It is now an abstract class that new SystemUI build targets should
  implement themselves. Implementations use this class to specify a
  root Dagger component used to stand up the rest of SystemUI.

  A default SystemUIInitializerImpl is provided that starts up AOSP.

- SystemUIAppComponentFactoryBase
  An abstract implementation of AppComponentFactory that is used to
  decide what SystemUIInitializer should be loaded.

  Implementers should return a SystemUIInitializer instance of their
  choice in #createSystemUIInitializer.

  The implementation of SystemUIAppComponentFactoryBase should in turn
  be specified in AndroidManifest.xml's
  Application.android:appComponentFactory attribute.

  The default SystemUIAppComponentFactory loads the
  SystemUIInitializer implementation reflectively. This should be
  considered deprecated behavior and may be removed in a future
  release. Please define your own SystemUIAppComponentFactoryBase
  implementation and return a specific SystemUIInitializer
  implementation from it.

Currently, AOSP, TV, and Pixel specific builds still rely on the
reflective loader. Other internal builds have been moved off of the
reflective loader.

Bug: 235624311
Test: manual
Change-Id: I7029fb2960f54fb29e08b96ee49170f150a1d0b6