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>

tunnel/src/main/java/com/wireguard/android/backend/GoBackend.java

@@ -5,6 +5,8 @@
 
 package com.wireguard.android.backend;
 
+import com.wireguard.util.NonNullForAll;
+
 import android.content.Context;
 import android.content.Intent;
 import android.os.Build;
@@ -30,6 +32,7 @@ import java.util.concurrent.TimeoutException;
 
 import java9.util.concurrent.CompletableFuture;
 
+@NonNullForAll
 public final class GoBackend implements Backend {
     private static final String TAG = "WireGuard/" + GoBackend.class.getSimpleName();
     private static CompletableFuture<VpnService> vpnService = new CompletableFuture<>();