TemplateParser: Pass FLAG_MUSTACHELOOKUP to enable parent context access

TemplateParser: Pass FLAG_MUSTACHELOOKUP to enable parent context access

This allows variables defined in an outer context to be used in inner
contexts. For example:

<h2>{{foo}}</h2>
<ul>
{{#things}}
    <!-- bar is a property of each thing, foo is an outer variable -->
    <li>{{foo}} is a {{bar}}</li>
{{/things}}
</ul>

Bug: T203209
Change-Id: Ib0ae0fb0b4be6b161f548c79db6fb6f4b831f7c1