adb: only submit USB writes on the worker thread.

After USB disconnection, io_submit will block until the endpoint comes
back up. We handle this in the worker thread by sending it a signal to
break it out of io_submit when we notice that USB has gone down, but
opportunistic writes from the main thread can get stuck in this scenario
as well. Submitting the writes only on the worker thread doesn't have a
measurable impact on performance, so avert this scenario by only
submitting writes from the worker thread.

Bug: http://b/157078255
Test: test_device.py
Change-Id: I1118f2e2a70d13f15592eb996e7084033ed5cb9d