ThemeChangeAwareActivity: reintroduce cache buster

This is still needed by certain icons, like the trash icon in the peer
editor.

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

app/src/main/java/com/wireguard/android/activity/ThemeChangeAwareActivity.java

@@ -6,14 +6,49 @@
 package com.wireguard.android.activity;
 
 import android.content.SharedPreferences;
+import android.content.res.Resources;
 import android.os.Bundle;
 import android.support.annotation.Nullable;
 import android.support.v7.app.AppCompatActivity;
 import android.support.v7.app.AppCompatDelegate;
+import android.util.Log;
 
 import com.wireguard.android.Application;
 
+import java.lang.reflect.Field;
+
 public abstract class ThemeChangeAwareActivity extends AppCompatActivity implements SharedPreferences.OnSharedPreferenceChangeListener {
+    private static final String TAG = "WireGuard/" + ThemeChangeAwareActivity.class.getSimpleName();
+
+    @Nullable private static Resources lastResources;
+    private static boolean lastDarkMode;
+    private static synchronized void invalidateDrawableCache(final Resources resources, final boolean darkMode) {
+        if (resources == lastResources && darkMode == lastDarkMode)
+            return;
+
+        try {
+            Field f;
+            Object o = resources;
+            try {
+                f = o.getClass().getDeclaredField("mResourcesImpl");
+                f.setAccessible(true);
+                o = f.get(o);
+            } catch (final Exception ignored) { }
+            f = o.getClass().getDeclaredField("mDrawableCache");
+            f.setAccessible(true);
+            o = f.get(o);
+            try {
+                o.getClass().getMethod("onConfigurationChange", int.class).invoke(o, -1);
+            } catch (final Exception ignored) {
+                o.getClass().getMethod("clear").invoke(o);
+            }
+        } catch (final Exception e) {
+            Log.e(TAG, "Failed to flush drawable cache", e);
+        }
+
+        lastResources = resources;
+        lastDarkMode = darkMode;
+    }
 
 
     @Override
@@ -31,10 +66,12 @@ public abstract class ThemeChangeAwareActivity extends AppCompatActivity impleme
     @Override
     public void onSharedPreferenceChanged(final SharedPreferences sharedPreferences, final String key) {
         if ("dark_theme".equals(key)) {
+            final boolean darkMode = sharedPreferences.getBoolean(key, false);
             AppCompatDelegate.setDefaultNightMode(
                     sharedPreferences.getBoolean(key, false) ?
                             AppCompatDelegate.MODE_NIGHT_YES :
                             AppCompatDelegate.MODE_NIGHT_NO);
+            invalidateDrawableCache(getResources(), darkMode);
             recreate();
         }
     }