[Pod] Split out SystemUI-application from SystemUI-core.

Splitting out the creation of System UI Application and its Dagger graph
from the rest of SystemUI-core will help us with modularization.

Any implementation code inside modules (com/android/systemui/pods/**)
is not part of SystemUI-core, which means changes to those files won't
require re-building the massive SystemUI-core target.

Bug: 307607958
Flag: EXEMPT refactor
Test: m SystemUI-application
Test: atest SystemUIApplicationImplTest
Test: atest SystemUITests
Test: smoke test of all of SysUI

Change-Id: Ibf655175abeb400526a31cd07c42c7a61fca39f5