Make flushing the broadcast loopers optional.

Some of the loopers in system_server process are
quite busy and it takes a while to reach the barrier
and the callers doesn't always need it. So, making
it optional so callers who actually need it can pass
an additional argument to the "wait-for-barrier" command
to use it.

Bug: 260158381
Test: atest services/tests/mockingservicestests/src/com/android/server/am/BroadcastQueueTest.java
Test: atest services/tests/mockingservicestests/src/com/android/server/am/BroadcastQueueModernImplTest.java
Change-Id: I4f23baf352b03919175662ddc85fafd95dc1598b