Moving followers slow change flag to FollowerStrategy

Part of clampers and ABS/ABC imorovement work. Strategy need to decide on slowChange and clamper could override it.

Bug: b/263362199
Test: atest FollowerBrightnessStrategyTest
Change-Id: Ib72cfe6b27b973f1d3e98cdeafdc2285d7d6f211