wait-for-broadcast-idle before stopUser in UserLifecycleTests.

Stopping a user heavily depends on broadcast queue, and that gets
crowded after user creation and switches. Which leads to a timeout
on stopping user. This CL makes sure broadcast queue is idle before
stopping a user, to prevent the timeout.

Bug: 200217686
Test: atest MultiUserPerfTests:android.multiuser.UserLifecycleTests --no-bazel-mode
Change-Id: I540a73d436eb28600e689c27e27ec6259d37e8b7