global: get rid of nonnull gradle hack

Hacking things up via gradle is not right, and package-info.java poses
problems with two modules, so instead we just apply it manually to every
class.

Remember to add this to new classes!

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

ui/src/main/java/com/wireguard/android/util/ClipboardUtils.java

@@ -5,6 +5,8 @@
 
 package com.wireguard.android.util;
 
+import com.wireguard.util.NonNullForAll;
+
 import android.content.ClipData;
 import android.content.ClipboardManager;
 import android.content.Context;
@@ -16,6 +18,7 @@ import android.widget.TextView;
  * Standalone utilities for interacting with the system clipboard.
  */
 
+@NonNullForAll
 public final class ClipboardUtils {
     private ClipboardUtils() {
         // Prevent instantiation