adb: reunregress waiting for device on start-server.

Move the invocation of adb_notify_device_scan_complete to
the end of device_connected, where we decrement connecting_devices.
Also, create a dedicated thread for handling hotplug events, instead of
reusing the main thread for this, since the main thread blocks until
device scan is complete.

Test: `adb kill-server; adb devices`
Change-Id: Ia73b1a57538174282a48ef73ab0a3e58152d6f83