Split HeadsUpManager implementation to HeadsUpManagerPhone

This CL splits HeadsUpManager with the basic functionality and the
phone (and car) related implementation. The former code leaves in
HeadsUpManager class, and the later code is moved to separated
HeadsUpManagerPhone class.

This contains the following minor changes:
- Move the utility static methods to HeadsUpUtil class.
- Chanege the return types of HeadsUpManager#getAllEntries() and
  HeadsUpManager#getTopEntry() from Collection<HeadsUpEntry> to
  Stream<NotificationData.Entry>.
- Add a private method: HeadsUpManagerPhone#getTopHeadsUpEntry()
- Make the mPluse propertes boolean instead of Collection in
  AmbientState and NotificationStackScrollLayout classes.
- Unify removeAllHeadsUpEntries() and releaseAllImmediately(), since
  they do same thing.
- Move getTopHeadsUpPinnedHeight method from HeadsUpManager to
  NotificationStackScrollLayout class, since only this class uses it.
- Add a simple test.

Bug: 63874929
Bug: 62602530
Test: Compile and ran "runtest systemui"
Change-Id: I3e5160803b9cdf1a164339557528ace0d35f8187