[flexiglass] Key platform theme color changes on assetsSeq instead.

This should trigger less often than LocalConfiguration.current.

Credit for the idea goes to kozynski@.

Fix: 442054578
Bug: 438964734
Test: Manually tested by changing the color scheme and observing they
 correctly update in the Quick Settings shade.
Flag: com.android.systemui.scene_container
Change-Id: I0b84158bd18e11914afd1ec5a01832fbcfbe4482