Add Mac NDK asset, and fetch NDK on Android compile bots.

infra/bots/recipes/swarm_trigger.expected/Build-Ubuntu-GCC-Arm64-Debug-Android_Vulkan.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\": \"Build-Ubuntu-GCC-Arm64-Debug-Android_Vulkan\", \"buildnumber\": 5, \"mastername\": \"client.skia\", \"path_config\": \"kitchen\", \"recipe\": \"swarm_trigger\", \"revision\": \"abc123\", \"slavename\": \"skiabot-linux-swarm-000\"}"
     ],
     "name": "print properties",
     "~followup_annotations": [
@@ skipping 253 matching lines @@
       "\nimport shutil\nimport sys\nshutil.copy(sys.argv[1], sys.argv[2])\n",
       "[SLAVE_BUILD]/skia/infra/bots/assets/android_sdk/VERSION",
       "/path/to/tmp/"
     ],
     "name": "read android_sdk VERSION"
   },
   {
     "cmd": [
       "python",
       "-u",
+      "\nimport shutil\nimport sys\nshutil.copy(sys.argv[1], sys.argv[2])\n",
+      "[SLAVE_BUILD]/skia/infra/bots/assets/android_ndk_linux/VERSION",
+      "/path/to/tmp/"
+    ],
+    "name": "read android_ndk_linux VERSION"
+  },
+  {
+    "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",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@python.inline@@@@",
       "@@@STEP_LOG_LINE@python.inline@import sys, os@@@",
       "@@@STEP_LOG_LINE@python.inline@path = sys.argv[1]@@@",
       "@@@STEP_LOG_LINE@python.inline@mode = int(sys.argv[2])@@@",
@@ skipping 87 matching lines @@
       "os:Ubuntu",
       "--tag",
       "revision:abc123",
       "--tag",
       "slavename:skiabot-linux-swarm-000",
       "--tag",
       "stepname:compile_skia on Ubuntu",
       "--idempotent",
       "--cipd-package",
       "android_sdk:skia/bots/android_sdk:version:0",
+      "--cipd-package",
+      "android_ndk_linux:skia/bots/android_ndk_linux:version:0",
       "[dummy hash for compile_skia]",
       "--",
       "--workdir",
       "../../..",
       "swarm_compile",
       "buildername=Build-Ubuntu-GCC-Arm64-Debug-Android_Vulkan",
       "mastername=client.skia.compile",
       "buildnumber=1",
       "slavename=skiabot-dummy-compile-slave",
       "reason=Triggered by Skia swarm_trigger Recipe",
@@ skipping 76 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
   }
 ]