debugd: Use DBus service constants defined in system_api. BUG=None TEST=`FEATURES=test emerge-$BOARD platform2` TEST=Verify that debugd runs on a device and answers DBus calls from Chrome. Change-Id: I81bda634ab17d33724d02325789caf81e24dbf8f Reviewed-on: https://chromium-review.googlesource.com/198862 Tested-by: Ben Chan <[email protected]> Reviewed-by: Jorge Lucangeli Obes <[email protected]> Commit-Queue: Ben Chan <[email protected]>