Add device_locked? method

[JaniJegoroff]

This PR provides implementation for #257

Test is implemented for Android only since iOS support is not implemented in Appium yet:
https://github.com/appium/appium/blob/master/lib/devices/ios/ios-controller.js#L1217

Android test results

Finished in 3 mins 55 secs

107 runs, 148 assertions, 0 failures, 0 errors, 0 skips

Appium console output

Janis-MacBook-Pro:android_tests janijegoroff$ arc
[1] pry(main)> device_locked?
false
[2] pry(main)> lock 5
nil
[3] pry(main)> device_locked?
true
[4] pry(main)> press_keycode 82
true
[5] pry(main)> device_locked?
false
[6] pry(main)>

[bootstraponline]

I think device_locked? is more descriptive. It also follows the pattern of the other end points a bit better. Other than that, the code looks good.

[JaniJegoroff]

Thanks, I will make review changes tomorrow.

[bootstraponline]

Thanks, please squash to one commit then it'll be ready to merge. Also on GitHub, there's no notification when a commit is pushed so after making a change it's a good idea to leave a comment so people know the PR has been updated.

[JaniJegoroff]

Done, changes squashed to one commit.

[bootstraponline]

Thanks! merged.