Bug 1571605 - Don't use file separator to find classes.jar. r=agi

I have landed bug 1527796 to support artifact build on Windows, but after bug 1515248 is landed, it is broken again. Path separator on Windows is '\', not '/'.  So we should use leaf name instead.

Differential Revision: https://phabricator.services.mozilla.com/D40758

--HG--
extra : moz-landing-system : lando

mobile/android/app/build.gradle

@@ -415,7 +415,7 @@ if (gradle.startParameter.taskNames.any { it.endsWith('UnitTest') }) {
 // Run two tasks, accepting the cost of duplicate work.
 android.applicationVariants.all { variant ->
     def jarTask = tasks["bundle${variant.name.capitalize()}Classes"]
-    def bundleJar = jarTask.outputs.files.find({ it.absolutePath.contains('/classes.jar') })
+    def bundleJar = jarTask.outputs.files.find({ it.name == 'classes.jar' })
 
     task("findbugsHtml${variant.name.capitalize()}", type: FindBugs) {
         outputs.upToDateWhen { false } // We always want to re-run findbugs when asked.