Trigger 64 bit android telemetry_perf_unittests on bots with 1 device.

10 shards with 20 min timeout. Sample shard:
https://chromium-swarm.appspot.com/task?id=377545056869e810

Bug: 670879
Change-Id: I8886aa6a2442532edeb8ad6bb1e0480938346ca3
Reviewed-on: https://chromium-review.googlesource.com/578447Reviewed-by: Ned Nguyen <nednguyen@google.com>
Commit-Queue: Benjamin Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488717}

testing/buildbot/chromium.android.json

@@ -1637,13 +1637,13 @@
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
             {
-              "android_devices": "6",
+              "android_devices": "1",
               "device_os": "MMB29Q",
               "device_type": "bullhead"
             }
           ],
           "hard_timeout": 1200,
-          "shards": 1
+          "shards": 10
         }
       }
     ]