[Status Bar Refactor] 2.1/N: Migrate ScrimController's

panelExpanisonChanged listener off of the
StatusBar.ExpansionChangedListener interface and onto
the PanelExpansionListener interface.

These two listener interfaces do the same thing, so we should unify them
into one interface. All 2.x CLs will migrate listeners to
the PanelExpansionListener interface.

Test: manual (verify ScrimController's callback still triggers)
Bug: 200063118
Change-Id: Ie0cec69a6571a936fa98be39e6de118cc0a6bdaf