Bump chrome_public_test_apk timeout by 10min on Kitkat Tablet Tester.

From 30m to 40m.

It's hovering pretty close to 30 min currently:
https://chromium-swarm.appspot.com/tasklist?c=name&c=state&c=created_ts&c=user&c=bot&c=duration&et=1506978420000&f=buildername%3AKitKat%20Tablet%20Tester&f=name%3Achrome_public_test_apk&l=50&n=true&q=dura&s=created_ts%3Adesc&st=1506892020000

R=dpranke@chromium.org

Bug: 
Change-Id: I6c050bdb6ca5a8528b132de5bdaad98ab6aeadbd
Reviewed-on: https://chromium-review.googlesource.com/695837Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Benjamin Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#505853}

testing/buildbot/chromium.android.json

@@ -2551,7 +2551,7 @@
             }
           ],
           "expiration": 10800,
-          "hard_timeout": 1800,
+          "hard_timeout": 2400,
           "output_links": [
             {
               "link": [