Restart B2G before each test

This restarts B2G before each test.  It adds ~25s to each test, so slows down the test run considerably.  It may be best to add this as a command-line option.

This shouldn't be merged as-is, it's more of a proof-of-concept.  This, combined with the way we call removePermission(), seems to remove the ability of insert_contact() to work correctly (it always errors out with a permissions problem).

The lockscreen is also a bit janky after restarting B2G.  I had to move the place where we unlock the lockscreen in order to get it to work correctly, although I don't know why.

We should also, if we decide to go with something like this, replace the time.sleep(15) with something a bit more deterministic, possibly combined with a smaller sleep.