[Systrace] Add support for on-demand stopping
Created by: derkling
Systrace can be "stopper on demand" by just running it on background and than sending an ENTER when we are done: https://github.com/catapult-project/catapult/blob/master/systrace/systrace/run_systrace.py#L169
The Android tests we integrated so far instead they specify a tracing time, which is quite overfilling for many experiments. We should better support this API in: https://github.com/ARM-software/lisa/blob/master/libs/utils/android/workload.py#L100