[GeckoView] Don't use the system fonts (bug 1882613)

On Android there's almost no chance we find a substitution font so
just use one of the font embedded in pdf.js.

web/app.js

@@ -1004,6 +1004,16 @@ const PDFViewerApplication = {
       AppOptions.set("docBaseUrl", this.baseUrl);
     }
 
+    // On Android, there is almost no chance to have the font we want so we
+    // don't use the system fonts in this case.
+    if (
+      typeof PDFJSDev === "undefined"
+        ? window.isGECKOVIEW
+        : PDFJSDev.test("GECKOVIEW")
+    ) {
+      args.useSystemFonts = false;
+    }
+
     // Set the necessary API parameters, using all the available options.
     const apiParams = AppOptions.getAll(OptionKind.API);
     const loadingTask = getDocument({