Temporarily move all chromium.tests.avd tasks to a template pool.

Bug: 812428
Change-Id: If15779eebc64714a2a8f27f45379317edbc513f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2088498Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747315}

testing/buildbot/mixins.pyl

@@ -490,7 +490,7 @@
       'dimensions': {
         'device_os': None,
         'device_type': None,
-        'pool': 'chromium.tests.avd',
+        'pool': 'chromium.tests.avd.template',
         'ssd': '1',
       },
       'named_caches': [
@@ -544,7 +544,7 @@
       'dimensions': {
         'device_os': None,
         'device_type': None,
-        'pool': 'chromium.tests.avd',
+        'pool': 'chromium.tests.avd.template',
         'ssd': '1',
       },
       'named_caches': [