Revert "android-pie-arm64-rel cq experiment to 100%."

This reverts commit ed064683.

Reason for revert: Running out of capacity, see https://crbug.com/1020376

Original change's description:
> android-pie-arm64-rel cq experiment to 100%.
> 
> Bug: 765833
> Change-Id: I9558db3934ebae0252f0fd4f2861c3c4441d7020
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1871639
> Auto-Submit: Andrew Luo <aluo@chromium.org>
> Reviewed-by: Aaron Gable <agable@chromium.org>
> Commit-Queue: Aaron Gable <agable@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#711093}

TBR=agable@chromium.org,aluo@chromium.org

Change-Id: I074d0a0dd94d53b6967496b7df42c9fa3ccf39e7
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 765833
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1894045Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Commit-Queue: Stephen Martinis <martiniss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711492}

infra/config/buckets/try.star

@@ -257,7 +257,7 @@ android_builder(
 android_builder(
     name = 'android-pie-arm64-rel',
     tryjob = tryjob(
-        experiment_percentage = 100,
+        experiment_percentage = 50,
     ),
 )
 

infra/config/generated/commit-queue.cfg

@@ -57,7 +57,7 @@ config_groups: <
       >
       builders: <
         name: "chromium/try/android-pie-arm64-rel"
-        experiment_percentage: 100
+        experiment_percentage: 50
       >
       builders: <
         name: "chromium/try/android_arm64_dbg_recipe"

infra/config/generated/cq-builders.md

@@ -332,7 +332,7 @@ as required builders.
   * Experiment percentage: 10
 
 * [android-pie-arm64-rel](https://ci.chromium.org/p/chromium/builders/try/android-pie-arm64-rel) ([definition](https://cs.chromium.org/search?q=package:%5Echromium$+file:/cq.star$+-file:/beta/+-file:/stable/+android-pie-arm64-rel)) ([matching builders](https://cs.chromium.org/search?q=+file:trybots.py+android-pie-arm64-rel))
-  * Experiment percentage: 100
+  * Experiment percentage: 50
 
 * [chromeos-kevin-experimental-rel](https://ci.chromium.org/p/chromium/builders/try/chromeos-kevin-experimental-rel) ([definition](https://cs.chromium.org/search?q=package:%5Echromium$+file:/cq.star$+-file:/beta/+-file:/stable/+chromeos-kevin-experimental-rel)) ([matching builders](https://cs.chromium.org/search?q=+file:trybots.py+chromeos-kevin-experimental-rel))
   * Experiment percentage: 5