adb: partially revert b5e11415.

Revert the write_msg_lock part of commit b5e11415. A write which hangs
will hold onto the mutex, preventing the device kick from ever
happening, which also causes lots of other stuff to hang, due to Kick
being called with the transport lock taken.

Test: python test_devices.py
Change-Id: Ie7c958799c93cad287c32d6bbef30c07f40c2d51