android support! thanks to TachiWeb devs.

AndroidCompat/src/main/java/com/f2prateek/rx/preferences/StringSetAdapter.java

@@ -0,0 +1,22 @@
+package com.f2prateek.rx.preferences;
+
+import android.annotation.TargetApi;
+import android.content.SharedPreferences;
+import android.support.annotation.NonNull;
+import java.util.Set;
+
+import static android.os.Build.VERSION_CODES.HONEYCOMB;
+
+@TargetApi(HONEYCOMB)
+final class StringSetAdapter implements Preference.Adapter<Set<String>> {
+  static final StringSetAdapter INSTANCE = new StringSetAdapter();
+
+  @Override public Set<String> get(@NonNull String key, @NonNull SharedPreferences preferences) {
+    return preferences.getStringSet(key, null);
+  }
+
+  @Override public void set(@NonNull String key, @NonNull Set<String> value,
+      @NonNull SharedPreferences.Editor editor) {
+    editor.putStringSet(key, value);
+  }
+}