Starting in Android M, there's an official way to unlock the screen when starting an emulator:
The command adb shell wm dismiss-keyguard will, as it says, dismiss the keyguard.

So we should use this for emulators running API 23 and above, and otherwise fallback to the existing not-so-reliable method.

(Source: https://plus.google.com/+TorNorbye/posts/1SpmREBVBy6)