Added unlock and lock methods for Android

Requires https://github.com/appium/appium-adb/pull/42 in order for lock to work.

If this looks good, I'll make issues on all the client drivers as well. 

I have started on a test as well, should it be added in to this pull or as another?
If you change this to 
```
if (req.device.unlock) {
  req.device.unlock(getResponseHandler(req, res));
} else {
  cb(new NotYetImplementedError(), null);
}
```
Then not only do you need not make any changes in ios.js, but firefoxos and any future device will also be covered.