adb: Handle adb connect in a thread

adb connect calls connect() in the event loop. If you pass a wrong ip
address or the server is slow to respond, this will block the event loop
and you can't even kill the adb server with adb kill-server. Handle connect
requests in a service thread instead.

Change-Id: I2ee732869a3dc22a6d3b87cf8ac80acaa7790037