Implement NotifBundler pluggable

BundleCoordinator hosts new pluggable NotifBundler
and sets it on ShadeListBuilder:

BundleCoordinator.attach
NotifPipeline.setNotifBundler
ShadeListBuilder.setBundler

ShadeListBuilder.setBundler creates top level list of BundleEntry

Bug: 396301289
Test: BundleCoordinatorTest
Flag: com.android.systemui.notification_bundle_ui
Change-Id: I3001c01f19679d567c53f65e26e43fca31cb3c29