Fix talkback issue of Caption preference

Talkback speaks "double-tap to activate" on the LayoutPreference in the
Caption preference page, but it doesn't have any action after double
tap. So it should be set to unclickable. Set android:selectable to
false to fix it.

Fixes: 141900530
Fixes: 141901515
Test: launch TB and test it by manually
Change-Id: I82720dc46085b84d188730545061269d110f9a7c

res/xml/captioning_settings.xml

@@ -25,6 +25,7 @@
         android:key="caption_preview"
         android:title="@string/summary_placeholder"
         android:layout="@layout/captioning_preview"
+        android:selectable="false"
         settings:searchable="false"/>
 
     <PreferenceCategory