[Status Bar Refactor] 1/N: Define a singleton PanelExpansionStateManager

and pipe existing panel expansion callbacks through it.

More code will be moved into PanelExpansionStateManager in future CLs --
that's why this CL is 1/N.

Also, #onQsExpansionChanged was never implemented by any listeners so
that method is removed.

Test: manual (verified all callbacks are still triggered)
Test: atest SystemUITests
Test: atest PanelExpansionStateManagerTest (new)
Bug: 200063118
Change-Id: Ic9587d5f335db6b720be01d7681f861f3922485c