adb: put legacy shell: service back in.

ddmlib does not use the ADB client, but instead connects directly to
the adb server. This breaks some of the assumptions I previously made
when enabling the shell protocol.

To fix this, the adb server now defaults to no protocol for the
standalone command, and the shell protocol must be explicitly requested
by the client. For example:
  shell:echo foo     -- no shell protocol
  shell,v2:echo foo  -- shell protocol

As long as I was touching the shell service arguments I also changed
them to no longer duplicate the command-line arguments. This allows
more flexibility to change the adb client CLI if necessary and makes
the code more readable.

Bug: http://b/24148636
Change-Id: I28d5ae578cf18cbe79347dc89cea1750ff4571a8