Don't dismiss adb authorization prompt upon USB disconnect.

adbd expects a reply from system_server for authorization: normally
there's a reply of "OK" or "NO", but when the prompt is dismissed
because of USB disconnect, no response is sent, and we can't prompt for
user authorization until system_server or adbd is restarted.

Dismissing the authorization prompt on USB disconnect is also wrong: the
request might not be coming in over USB.

Bug: http://b/145814587
Test: unplugged a device with authorization dialog open
Change-Id: I5a9925b917a7169213256daa557637b770583058