Add new "shell command" feature to Binder objects.

IBinder has a new common interface for sending shell commands
to it.  This can be implemented by system services to provide
a shell interface to the service, without needing to have separate
shell java code.

Also add a new "cmd" command line tool, which invokes the shell
command method on a system service.  This is much like dumpsys,
but for shell commands.

Change-Id: I95dc80c881a28cefb76957ad4d4fd9b3ed9630df