adb: tell the client what transport it received.

Prerequisite for making `adb root` wait for the device that it told to
restart to disappear: the client needs to know which transport to wait
on.

Bug: http://b/124244488
Test: manual
Change-Id: I474559838ad7c0e961e9d2a98c902bca3b60d6c8
5 files changed