ng-attr-<attrName>
Support for conditionally declaring an HTML attribute is included with Angular as the dynamically-titled ng-attr-<attrName> directive.
Official Docs for ng-attr
Example
In your case, the code might look like this:
<li
id="{{template._id}}"
class="template-box"
type="template"
ng-repeat="template in templates"
ng-attr-draggable="dragSupported() === true"
></li>
Demo
JSFiddle
This contains examples of usage for the following values: true, false, undefined, null, 1, 0, and "". Note how typically-falsey values may yield unexpected results.