Fixed screen resizing upon first screen touch for pre android 4.4 devices

The new native activity class introduced in 3.0 sets these flags:

```
View.SYSTEM_UI_FLAG_HIDE_NAVIGATION | View.SYSTEM_UI_FLAG_FULLSCREEN
```

This does hide the navigation bar, however for devices running pre android 4.4:

*'With this approach, touching anywhere on the screen causes the navigation bar (and status bar) to reappear and remain visible. The user interaction causes the flags to be be cleared.'*
https://developer.android.com/training/system-ui/navigation.html

When this happens the vertical screen size of the native activity is reduced to fit the navigation bar.

My fix is to not set these flags for pre 4.4 devices so that the navigation bar always appears which also prevents any vertical screen cropping.

I also bumped the Build.VERSION.SDK_IN check from 18 to 19 since the documentation says that's when the immersive feature was introduced:

*'Android 4.4 (API Level 19) introduces a new SYSTEM_UI_FLAG_IMMERSIVE flag...'*
https://developer.android.com/training/system-ui/immersive.html