Adds SYSUI_STATE_BUBBLES_EXPANDED to QuickStepContract.

Also, modifies bubbles to update this value when bubbles are expanded or collapsed.

This will work in concert with the other CLs in this topic to collapse bubbles (but not the app behind it) when a swipe-to-home gesture occurs. It will not affect the behavior of the home button in three-button nav.

Test: atest SystemUITests
Bug: 146167884
Change-Id: I825afc7fa58c66e92a277bf94876c23917c412da