Split NotificationViewHierarchyManager out of StatusBar.

NotificationViewHierarchyManager handles bundling and unbundling of
notifications. In doing so, which notifications are parents/children of
the other can change. NotificationViewHierarchyManager makes sure the
view hierarchy of the notifications matches their grouping.

Bug: 63874929
Bug: 62602530
Test: runtest systemui
Test: Compile and run
Change-Id: Ia1c8ed75d4eb8df52897c5d6aa0713f8335b2a19