On Android show the system UI when an SDL window is windowed, hide the system UI when it's fullscreen, like we do on iOS.

We're increasing the Android SDK minimum version to API 19, this doesn't increase the minimum target API, which is API 14.

android-project/app/build.gradle

@@ -8,14 +8,14 @@ else {
 }
 
 android {
-    compileSdkVersion 16
+    compileSdkVersion 19
     buildToolsVersion "26.0.1"
     defaultConfig {
         if (buildAsApplication) {
             applicationId "org.libsdl.app"
         }
         minSdkVersion 14
-        targetSdkVersion 16
+        targetSdkVersion 19
         versionCode 1
         versionName "1.0"
         externalNativeBuild {
@@ -65,4 +65,4 @@ dependencies {
         exclude group: 'com.android.support', module: 'support-annotations'
     })
     testCompile 'junit:junit:4.12'
-}
+}