[resultdb] enable ResultDB on chromium.android at 30%

https://crrev.com/c/2582808 enabled ResultDB for junit_tests
on chromium.FYI at 30%. However, it turned out that
- android-code-coverage is the only builder with junit_tests in
chromium.fyi, and
- the builder has been in a very unhealthy status for a long time
already.
: https://ci.chromium.org/p/chromium/builders/ci/android-code-coverage?limit=100

Therefore, chromium.FYI cannot be used to test ResultDB for junit tests.
Instead, I replicated a builder from Android arm Builder (dbg) and set
the experiment flag to test ResultDB on chromium.android junit_tests.
: http://shortn/_pZLcaLnZ47

It WAI and results were streamed to ResultDB successfully.
This CL sets the junit experiment flag to 30% for chromium.android.

R=gbeaty@chromium.org,chanli@chromium.org

Bug: 1084332
Change-Id: Ib8db8d46cc4d864b9a6de81f5fd4b0c280913f7e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2586248Reviewed-by: Chan Li <chanli@chromium.org>
Reviewed-by: Garrett Beaty <gbeaty@chromium.org>
Commit-Queue: Scott Lee <ddoman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836133}

infra/config/lib/ci.star

@@ -506,6 +506,9 @@ def android_builder(
         builder_group = "chromium.android",
         goma_backend = builders.goma.backend.RBE_PROD,
         goma_jobs = goma_jobs,
+        experiments = {
+            "chromium.resultdb.result_sink.junit_tests": 30,
+        },
         **kwargs
     )