Adding "attach-agent" to ActivityManagerShellCommand

This new command is used to attach runtime agents to a running application:

attach-agent <PROCESS> <FILE>
   Attach an agent to the specified <PROCESS>,
   which may be either a process name or a PID.

Test: m test-art-host, manual testing:
    . invalid syntax, missing arguments
    . invalid syntax, extra arguments
    . invalid numeric PID
    . invalid process name
    . valid process, not debuggable
    . valid process, missing agent
    . valid process, valid agent

Bug: 31682382

Change-Id: Ife88dbf23991dde7945d9208e54cd014bb7ecdc6