Run chrome_public_unit_test_apk on crosshatch instead of walleye

The change is made for the CI builder "ci/android-pie-arm64-rel" and
the mirrored CQ "try/android-arm64-rel".

The estimator shows 38 devices are needed to support this test suite:
http://shortn/_SPz2HLRgJi

The crosshatch pool is quite capable to handle this:
http://shortn/_YLeqNYR93b

Bug: 1475086
Change-Id: Ide31c280a8842251a8167f7e5a5f8c9f81ff10b6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4942867
Commit-Queue: Haiyang Pan <[email protected]>
Reviewed-by: James Shen <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1210552}
diff --git a/testing/buildbot/test_suite_exceptions.pyl b/testing/buildbot/test_suite_exceptions.pyl
index 435a0f6..080e34fd 100644
--- a/testing/buildbot/test_suite_exceptions.pyl
+++ b/testing/buildbot/test_suite_exceptions.pyl
@@ -1614,6 +1614,9 @@
       },
       'android-pie-arm64-rel': {
         'swarming': {
+          'dimensions': {
+            'device_type': 'crosshatch',
+          },
           # TODO(crbug.com/1462745): Temporarily increase the shard to 3 for the network issue.
           'shards': 3,
         },