Add Swarming task for downloading Android SDK on Skia bots

scripts/slave/recipes/skia/swarm_trigger.expected/legacy_android_sdk.json

 [
   {
     "cmd": [
       "git",
       "remote",
       "set-url",
       "origin",
       "https://skia.googlesource.com/skia.git"
     ],
     "cwd": "[SLAVE_BUILD]/skia",
@@ skipping 385 matching lines @@
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
       "\nimport shutil\nimport sys\nshutil.copy(sys.argv[1], sys.argv[2])\n",
       "[SLAVE_BUILD]/skia/infra/bots/android_sdk_hash",
       "/path/to/tmp/"
     ],
-    "name": "Read android_sdk_hash",
-    "~followup_annotations": [
-      "step returned non-zero exit code: 1",
-      "@@@STEP_EXCEPTION@@@"
-    ]
+    "name": "Read android_sdk_hash"
   },
   {
     "cmd": [
       "python",
       "-u",
       "\nimport sys, os\npath = sys.argv[1]\nmode = int(sys.argv[2])\nif not os.path.isdir(path):\n  if os.path.exists(path):\n    print \"%s exists but is not a dir\" % path\n    sys.exit(1)\n  os.makedirs(path, mode)\n",
       "[SLAVE_BUILD]/swarming_temp_dir",
       "511"
     ],
     "name": "makedirs swarming tmp dir (2)",
@@ skipping 219 matching lines @@
       "@@@STEP_LOG_END@json.output@@@",
       "@@@STEP_LINK@shard #0 isolated out@blah@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]