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?
Style guide says to avoid bracketless one-liners.  Looks like that hasn't been enforced consistently, but since I also prefer it that way, I'll appeal to authority in this case: please add brackets. ;)