[Status Bar Refactor] 3/N: Move PanelBar#onPanelExpansionChanged into

PanelExpansionStateManager.

Added bonuses/wins in this CL:
 - We no longer pass PhoneStatusBarView into PanelViewController.
 - Notification-panel-related code is no longer piped through the
 unrelated status bar view.
 - PanelBar is deleted! It was an abstract superclass with only 1
 implementation, and it was unrelated to notification panels even though
 it has "panel" in the name, so it's a huge win to be able to delete
 this class.
 - The panel expansion changed code is now tested.

Test: atest PanelExpansionStateManagerTest
Test: Verified debug logs in PanelExpansionStateManager are the same as
they were previously.
Bug: 200063118

Change-Id: I4809edaaa7fdadc0d6cc104e447935ecbb42f472