[Pod] Split ContextInitializer into 2 separate interfaces.

ContextInitializer is used to provide Context to parts of
SysUI that are created before the Context is available. The
two parts of SysUI that implement ContextInitializers are:
 - The main application (SystemUIApplication)
 - Content providers

This CL splits ContextInitializer into
ApplicationContextInitializer and
ContentProviderContextInitializer.

The advantage is that ContentProviderContextInitializer
doesn't need access to SystemUIInitializer, so we can remove
that from the ContentProviderContextInitializer. This will
let us split out SysUI's Dagger graph construction out from
the rest of the SysUI monolith.

Bug: 307607958
Flag: EXEMPT refactor
Test: mp sysuig -> verify SystemUIApplication and all
content providers are still started via logs
Test: atest SystemUIApplicationTest CustomizationProviderTest

Change-Id: I9f1216a2f65b2991fc0696359b5aa5f0e6ac61fc