Use SystemUtil.waitForBroadcasts.

This uses wait-for-broadcast-barrier on U+ and is supposedly faster
while providing similar guarantees. We also don't need to roll out our
own and can depend on this implementation instead.

Bug: 226091780
Test: atest CtsSafetyCenterTestCases
Change-Id: I9171e3a12c01da1612440bfa72558dae0c9d4c14