Add support for android immsersive mode

Google introduced the "immersive mode" in Android API 19 which hides the navigation and notification bar (and frees up some space).

[For more information about the immersive mode read this](https://developer.android.com/training/system-ui/immersive.html)

![immersive mode](https://developer.android.com/images/training/imm-states.png)

This change is currently only available when you add the following line to your lime-project file:

    <android target-sdk-version="19" />

**Note:** That this change only affects devices using API level 19+, older Android versions will ~~obviously~~ should still work.

There is a related pull request in openfl-native: https://github.com/openfl/openfl-native/pull/175