UsbDebuggingManager : Handle race condition when toggling rapidly

When toggling adb enable/disable rapidly, you can have the situation where the thread's
run() gets invoked but mAdbEnabled is already set to false, hence listenToSocket() never even
gets invoked in which case mOutputStream/mSocket are both null. This patchset adds checks
around that.

Change-Id: Iba56e7409435e22d690432fa663bc0a04a1a447d
(cherry picked from commit 99335b98df32928e86c1f00d99512632d7b7f8c3)