UsbDebuggingActivity: notify failure in onDestroy, not onStop.

On initial boot, the prompt can be shown and then get hidden, which
would previously cause the initial public key authorization request to
be spuriously rejected, if the adb client was running during boot. Move
the rejection to onDestroy instead, so we only reject prompts that have
no chance of being actually accepted.

Bug: http://b/159061108
Test: manual
Change-Id: I9acb1145b1f8a6c8d6f82d1c4459bcd86eb306df