Explicitly add "flush-broadcast-loopers" option.

Recently, flushing the broadcast loopers behavior, when
"wait-for-broadcast-barrier" command is invoked, is made
optional which resulted in some regressions in multi-user
tests as they same to be relying on flushing the broadcast
loopers behavior. So, explicitly add this option when
invoking the command.

Bug: 282848750
Test: atest android.multiuser.UserLifecycleTests
Change-Id: I6ba4e0858a63653b731c7a24b600ce9af758d39c