[Android] Raise chrome_public_test_apk timeout on chromium.linux:Android Tests (dbg).

BUG=715972
TBR=dpranke@chromium.org

Review-Url: https://codereview.chromium.org/2847783002
Cr-Commit-Position: refs/heads/master@{#468189}

testing/buildbot/chromium.linux.json

@@ -1565,7 +1565,7 @@
               "device_type": "hammerhead"
             }
           ],
-          "hard_timeout": 1800,
+          "hard_timeout": 2400,
           "output_links": [
             {
               "link": [