[Status Bar Refactor] Move PanelBar#updateScrimFraction into

ScrimController.

No need for PanelBar to calculate scrim values and notify
ScrimController of them, when ScrimController can just calculate them
directly.

Test: Manually verifying scrim fraction is still updated correctly via
logs
Bug: 200063118

Change-Id: I37df95015002825adf7217c4144950e85485af84
Merged-In: I37df95015002825adf7217c4144950e85485af84