Refactor shell_service.cpp to remove dependencies on service.cpp and
combine some common logic between PTY and raw subprocesses.
This will make it easier to add additional common code paths for
the upcoming shell protocol.
Change-Id: I497d30dd388de61b6e68d9086dce38f33dd92876
Upcoming changes to the shell will require significant additions to
the subprocess code, and it will be cleaner if it's in a separate file.
The only functional change here is a new debug tag specifically for
the shell service. Everything else has been copied exactly as-is in
order to make it easier to determine what's changing in upcoming CLs.
Change-Id: I13bd4294059051ee10e0d0c6a06affd8eca62967