feat(book/cache): add export picture option

app/src/main/res/values/strings.xml

@@ -994,6 +994,8 @@
     <string name="bitmap_cache_size_summary">Current cache max size %1$s MB</string>
     <string name="bitmap_cache_size">bitmap cache size</string>
     <string name="restart_to_apply_changes">Restart to apply changes</string>
+    <string name="export_pics_file">Export Picture Files</string>
     <!-- string end -->
 
+
 </resources>