Perf collector indiscriminately kills processes

[douglas-raillard-arm]

The PerfCollector class uses the following pattern:

    def reset(self):
        self.target.killall(self.perf_type, as_root=self.target.is_rooted)

This is not great as self.perf_type can be set to perf, so the class ends up indiscriminately killing all perf processes on the system, possibly as root. This is not a very good neighborly behavior. Instead, it should spawn a process (e.g. with Target.background()) and kill that process specifically, without interfering with resources it does not own.

[douglas-raillard-arm]

As BackgroundCommand.send_signal() now supports all signals (rather than just SIGKILL), it should be suitable for use in PerfCollector. The FtraceCollector can be taken as an example of how to use devlib-signal-target and send_signal().