[Status Bar Refactor] Remove some indirection about minFraction between

NotificationPanelViewController and PanelBar.

The current code flow is NPVC#setPanelScrimMinFraction =>
PanelBar#onPanelMinFractionChanged =>
NPVC#setMinFraction.

The minFraction doesn't need to flow through PanelBar if it's going to
go right back to NPVC.

Test: Test steps from ag/15759151
Bug: 200063118
Change-Id: If31a0d78cf4ac4a4d4bc6fcc7423a8006ab309fa