adb: move shell service to a separate file.

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