Wait for broadcast barrier to speed up tests.

For tests interesting in knowing if a broadcast has been sent, we
have a new "am wait-for-broadcast-barrier" command which can be
used to confirm that any broadcasts enqueued before that instant in
time have been scheduled for delivery.

Adding this "barrier" helps speed up tests and decrease flakiness.

Bug: 253906105
Test: TH
Change-Id: I8feee75102d353980d6ec94068f60fe00de429d9