Pipe appTransition signal to taskbar to pause region sampling when it's ongoing

For nav bar, RegionSamplingHelper calls LightTransitionsController#setIconsDark to apply color changes, which listens to mTransitionPending signal. For taskbar, due to the RegionSamplingHelper in Launcher and LightTransitionsController in SysUI, we need to pipe this signal (mTransitionPending) through.

Bug: 361593564
Test: https://android-build.corp.google.com/builds/abtd/run/L78300030006280283
Flag: EXEMPT bugfix
Change-Id: I4a3607ddc6c92ee1bee6464d121024582aa70870