uxperf workloads: only broadcast media mounted if needed
uxperf workloads: only broadcast media mounted if needed

Previously AndroidPerfWorkload broadcast media mounted unconditionally
inside push_assets and delete_assets. This change makes it so the
broadcast only happens if something was actually pushed/needs to be
delete.

jimboatarm/workload-automation,jimboatarm/workload-automation,jimboatarm/workload-automation,bjackman/workload-automation,bjackman/workload-automation,bjackman/workload-automation,jimboatarm/workload-automation,bjackman/workload-automation,jimboatarm/workload-automation,jimboatarm/workload-automation,bjackman/workload-automation,bjackman/workload-automation