Bug 1412460 - Move code for invoking build system out of mach_commands.py; r=mshal

The goal is to move this lower-level code somewhere where it can be
reused.

The only meaningful code change as part of the move was handling
of the mach context. Before, we could access self._mach_context.
After, we need to pass it as an argument. This also uncovered a bug
where "telemetry_handler" could be None but we called it
unconditionally. We now guard this call with an "if."

MozReview-Commit-ID: FWw4l6H1ayS

--HG--
extra : rebase_source : f6bf25eb8c52df2d09e4d5eddd5b85b38502e661