support PreferenceCategory without title.

<?xml version="1.0" encoding="utf-8"?>

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<LinearLayout

    xmlns:android="http://schemas.android.com/apk/res/android"

    android:layout_width="match_parent"

    android:layout_height="wrap_content"

    android:baselineAligned="false"

    android:layout_marginTop="16dp">

</LinearLayout>