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/widget/ToggleSwitch.java

@@ -6,12 +6,15 @@
 
 package com.wireguard.android.widget;
 
+import com.wireguard.util.NonNullForAll;
+
 import android.content.Context;
 import android.os.Parcelable;
 import androidx.annotation.Nullable;
 import android.util.AttributeSet;
 import android.widget.Switch;
 
+@NonNullForAll
 public class ToggleSwitch extends Switch {
     private boolean isRestoringState;
     @Nullable private OnBeforeCheckedChangeListener listener;