You can include favicon at your custom app clientlibs. Follow below steps:
Step 1:
You need to add favicon at your custom app clientlibs and this clientlib should be reffered from template policy resources folder
Step 2 :
If you are using WCM Core Page component then it will work as shown in
https://github.com/adobe/aem-core-wcm-components/blob/e49999b3733ceb487fd40774ac4a15f921713336/conte...
<sly data-sly-test.appResourcesPath=${page.appResourcesPath} data-sly-call="${headResources.favicons @ path = appResourcesPath}"></sly>
OR you can also add favicon in DAM and make it authorable at sling site config and use same sightly in head.html
<link rel="icon" href="${pagemodel.faviconSource}"/>