merge adb stdout/stderr to work around subprocess bug (fixes #460)

This change merges the stdout/stderr streams for the adb subprocess calls to work around a subprocess bug in which subprocess doesn't notice that one of the streams has closed after the subprocess exits, so it never calls its *done* handler, and the adb module never starts tracking devices.

I see the problem consistently the first time adb is invoked to start the server with `adb start-server`, which actually starts the server; but not the second time, which doesn't start the server because the server is already started (hence issue #460).

This pull also includes a few improvements to adb/subprocess logging.