Allow adbd to notify systemd on startup

Using adbd through USB (on devices that support it) requires first creating the USB gadget configuration, then starting adbd and waiting for it to bind to the ConfigFS interface, and finally activating the USB gadget.

In order to simplify this process, this patch makes adbd use sd_notify() so we can provide a systemd service taking care of all the USB gadget configuration, and activate it only once adbd is ready.