Create adb Python package.

This is mostly just the AdbWrapper that we used in our tests, but I've
cleaned up the API to be a little more Pythonic (mostly in the sense
that commands are passed as lists rather than strings that are
shlex.split() by the shell command), and implemented the workaround
error checking for adb shell.

Move the tests up a directory. Having them buried a level down has
only been annoying.

There are now two files containing Python tests. test_device.py
contains tests specifically checking the AndroidDevice API, and
test_adb.py checks the ADB client program. To run both, use

    python -m unittest discover [-v]

Change-Id: Ibd158c528d31126a5b048bd00bc93039dbc468bc