usb:gadget:android: avoid sending extra uevents

Don't schedule a uevet from android_disconnect
if there is no change in the connection state
and if the DISCONNECT event was already sent.
This patch will avoid these unecessary messages:

android_work: did not send uevent (0 0   (null))

Change-Id: I54d66ef84fc19791da94d32f82bf6e5dcef3786d