Start region sampling once app transition starts, instead of when it finishes

Follow up to https://googleplex-android-review.git.corp.google.com/q/topic:%22region_sampling_app_transitioning%22

So app transition cycle runs in the order of pending -> starting -> finished or cancelled.

For nav bar, region sampling is paused between pending and starting instead of the entire cycle of pending to finished or cancelled. https://source.corp.google.com/h/googleplex-android/platform/superproject/main/+/main:frameworks/base/packages/SystemUI/src/com/android/systemui/statusbar/phone/LightBarTransitionsController.java;l=193;drc=c1274bdd5301b74ef502f9416f7df6cc3e37c018 For more information

The current implementation has a color change delay when swiping home if dark theme is turned off.

Bug: 361593564
Test: check the swipe home animation in light theme
Flag: EXEMPT bugfix
Change-Id: I693891e3cd8a2855a44e6dff9c4353e82e0bbb3b