BroadcastQueue: fix proto dump regression.

The underlying test here is assuming that a broadcast with an empty
list of receivers is still listed in the dump output, so factor out
notifyFinishBroadcast() to call in this situation.

Also replace flaky sleep() with "wait-for-broadcast-barrier".

Bug: 256174447
Test: atest CtsIncidentHostTestCases:ActivityManagerIncidentTest#testDumpBroadcasts
Change-Id: If60bcfff3f0509e976dec2842dce77c5412d5505