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/preference/VersionPreference.java

@@ -5,6 +5,8 @@
 
 package com.wireguard.android.preference;
 
+import com.wireguard.util.NonNullForAll;
+
 import android.content.ActivityNotFoundException;
 import android.content.Context;
 import android.content.Intent;
@@ -22,6 +24,7 @@ import com.wireguard.android.backend.WgQuickBackend;
 
 import java.util.Locale;
 
+@NonNullForAll
 public class VersionPreference extends Preference {
     @Nullable private String versionSummary;