Specify the Android build number as a swarming dimension

infra/bots/recipes/swarm_trigger.expected/Test-Android-GCC-Nexus7v2-GPU-Tegra3-Arm7-Release.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "\nimport json\nimport sys\n\nwith open(sys.argv[1]) as f:\n  content = json.load(f)\n\nprint json.dumps(content, indent=2)\n",
       "{\"buildername\": \"Test-Android-GCC-Nexus7v2-GPU-Tegra3-Arm7-Release\", \"buildnumber\": 5, \"mastername\": \"client.skia\", \"path_config\": \"kitchen\", \"recipe\": \"swarm_trigger\", \"revision\": \"abc123\", \"slavename\": \"skiabot-linux-swarm-000\"}"
     ],
     "name": "print properties",
     "~followup_annotations": [
@@ skipping 609 matching lines @@
       "test_skia/Android/def456/Test-Android-GCC-Nexus7v2-GPU-Tegra3-Arm7-Release/5",
       "--dump-json",
       "/path/to/tmp/json",
       "--expiration",
       "72000",
       "--io-timeout",
       "2400",
       "--hard-timeout",
       "14400",
       "--dimension",
+      "device_os",
+      "M",
+      "--dimension",
       "device_type",
       "flo",
       "--dimension",
       "os",
       "Android",
       "--dimension",
       "pool",
       "Skia",
       "--tag",
       "allow_milo:1",
@@ skipping 349 matching lines @@
       "@@@STEP_LINK@shard #0 isolated out@blah@@@",
       "@@@STEP_LINK@view steps on Milo@https://luci-milo.appspot.com/swarming/task/148aa78d7aa0000@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]