adb: fix deadlock.

libusb hotplug callbacks are called with the libusb lock taken, and we
call into libusb with our local mutex, so we need to enforce an ordering
between the two. Instead of calling device_connected or
device_disconnected directly, enqueue them onto the main thread.

Bug: http://b/62200735
Test: manually hotplugged a device
Change-Id: Ic5d55db83b47f4bb60f124ce94ddfe06f5f1a0c6