Use TYPE_APPLICATION_OVERLAY on Android O

See https://developer.android.com/about/versions/oreo/android-8.0-changes.html#all-aw

Closes GH-1

Author: saschpe

textoverlay/src/main/java/saschpe/android/textoverlay/service/TextOverlayService.java

@@ -92,10 +92,17 @@ public void onCreate() {
         textView.setTextColor(ContextCompat.getColor(this, colorId));
         textView.setText(lastUsedOverlayText);
 
+        int type;
+        if (android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.O) {
+            type = WindowManager.LayoutParams.TYPE_APPLICATION_OVERLAY;
+        } else {
+            type = WindowManager.LayoutParams.TYPE_SYSTEM_OVERLAY;
+        }
+
         WindowManager.LayoutParams params = new WindowManager.LayoutParams(
                 WindowManager.LayoutParams.WRAP_CONTENT,
                 WindowManager.LayoutParams.WRAP_CONTENT,
-                WindowManager.LayoutParams.TYPE_SYSTEM_OVERLAY,
+                type,
                 WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE |
                         WindowManager.LayoutParams.FLAG_NOT_TOUCHABLE,
                 PixelFormat.TRANSLUCENT);