usb: gadget: android: follow proper order on disconnect

We should composite_disconnect *before* updating our internal
state and sending uevent to userspace.
Also, hold cdev->lock while updating internal state.

Change-Id: If66a5a4f7d4a102a44aefc3ce4a154a5a1517dcf
Signed-off-by: Dima Zavin <dima@android.com>