Added --help option to dumpsys.

When invoked with --help, dumpsys will print:

usage: dumpsys
         To dump all services.
or:
       dumpsys [--help | -l | SERVICE [ARGS]]
         --help: shows this help
         -l: only list services, do not dump them
         SERVICE: dumps only service SERVICE, optionally passing ARGS to it

BUG: 26639621
Change-Id: Ic7f7a5ec80fb3da1dc5a412b07ef2859d31b09d2