[Status Bar Refactor] Remove thisFrac from PanelBar#panelExpansionChanged.

`thisFrac` and `frac` always had the same values when logged in
different scenarios, which makes sense: PanelViewController sets its
fraction internally, then calls #panelExpansionChanged with that value.
`thisFrac` is unnecessary, so this CL removes it.

Test: atest+manual
Bug: 200063118
Change-Id: I48d8dc273815f9d5907ee78be0a801b4dc9234b1