Remove support for Android API < 23 in WindowOverlayCompat

Summary:
Since minsdk version was increased to 23, we are deleting code using Android APIs < 23 for class WindowOverlayCompat

chnagelog: [Android][Breaking] Remove support for Android API < 23 in WindowOverlayCompat

Differential Revision: D48545505

fbshipit-source-id: 7cc85c60a801c02f53e868a397c84f4dfb379348

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/devsupport/WindowOverlayCompat.java

@@ -16,15 +16,10 @@
  */
 /* package */ class WindowOverlayCompat {
 
-  private static final int ANDROID_OREO = 26;
   private static final int TYPE_APPLICATION_OVERLAY = 2038;
 
-  static final int TYPE_SYSTEM_ALERT =
-      Build.VERSION.SDK_INT < ANDROID_OREO
-          ? WindowManager.LayoutParams.TYPE_SYSTEM_ALERT
-          : TYPE_APPLICATION_OVERLAY;
   static final int TYPE_SYSTEM_OVERLAY =
-      Build.VERSION.SDK_INT < ANDROID_OREO
+      Build.VERSION.SDK_INT < Build.VERSION_CODES.O
           ? WindowManager.LayoutParams.TYPE_SYSTEM_OVERLAY
           : TYPE_APPLICATION_OVERLAY;
 }