Perform NLVM combines on the bg thread

It seems like although the flows themselves don't emit too often (thanks
to distinctUntilChanged), the combines still do quite a bit of work that
should ideally happen in the background. The biggest offender here is
the footer visibility, as it combines a lot of flows that can emit
relatively often.

Also reducing the amount of emissions in the shadeExpansion sub-flows
using distinctUntilChanged. Removed qsExpansion as it's redundant since
we're already checking qsFullScreen, which is only true when qsExpansion
is 1.

Bug: 326018643
Bug: 324483341
Test: v2/android-crystalball-eng/health/microbench/systemui/main/systemui-notification-4-jank-suite
Test: v2/android-crystalball-eng/health/microbench/systemui/systemui-notification-onmeasure-jank-suite
Flag: NONE

Change-Id: Ifc470bd1bbf9142c1114d7bdfb92449be5f754ce