Bug 1270543 - Part 2: Enable choosing an app by double tapping in the download prompt. r=liuche

This tells the app chooser dialog that appears when downloading a file to use the "Just once" button when two successive taps in a row on the same app have been detected.

MozReview-Commit-ID: Iejs1ROvt6n

--HG--
extra : transplant_source : %D3Y%C4%5D%DB%BC%26%C1Tr%8D%82%1Cmy%A5B%08g%D8
extra : histedit_source : e0e0eb9e7d0a64d86384515c2d93ac527d45e967

mobile/android/components/HelperAppDialog.js

@@ -206,7 +206,9 @@ HelperAppLauncherDialog.prototype = {
       buttons: [
         bundle.GetStringFromName("helperapps.alwaysUse"),
         bundle.GetStringFromName("helperapps.useJustOnce")
-      ]
+      ],
+      // Tapping an app twice should choose "Just once".
+      doubleTapButton: 1
     }, (data) => {
       if (data.button < 0) {
         return;