Ignore system gestures while bubbles are expanded.

This flag is set by System UI in ag/10760162, when bubbles are expanded. We want to disallow system gestures from starting when bubbles are expanded, since otherwise a swipe up will both close the bubbles, and the app behind them.

Test: install launcher, add bubbles, open an app, expand bubbles, observe that swiping up only closes bubbles
Test: same but in three button nav, home button should both close bubbles + the app behind it
Bug: 146167884
Change-Id: I27ed225d2029e3cb82f4959739c7b6f25fe8adb7