Add adbconnection plugin

This plugin will take care of creating and managing a debugger connection
through the adb daemon. This involves sending DDMS messages from the
runtime, loading the JDWP agent when required, and setting up the
connection. We need this since DDMS packets can be sent even if there has
not been a full handshake done with the JDWP agent.

Add an 'adbconnection' value to '-XjdwpProvider:...' to allow one to
request that the adbconnection plugin be used to provide JDWP
functionality.

Bug: 62821960
Test: Manual, Flash walleye, debug apps

Change-Id: Id9bed589b7c5e3830e6cdfbfee460b091459a27b