PandoraServer: submit intents in the subscriber scope

Also limit parallelism to one thread to make sure all flow listeners are
actually getting all the intents.

Bug: 290204719
Bug: 290204300
Bug: 290204634
Test: atest avatar -- --atest-include-filter "avatar:SecurityTest"
Change-Id: Iaf5fbec0db06f918406aec8e65661438ccfb0918