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?
yes, we allow them specifically in this shortuct-to-a-callback instance that's an overwhelmingly common idiom.