[infra] Reduce Android shard count.

When the shard count is greater than the number of machines in the pool, shards are guarenteed to wait, increase latency relative to a lower shard count.

Change-Id: I1fd61a59837b767748c13120dec30ca6eb352f28
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/450946
Reviewed-by: Alexander Aprelev <[email protected]>
Commit-Queue: Ryan Macnak <[email protected]>

Author: rmacnak-google

tools/bots/test_matrix.json

@@ -806,7 +806,7 @@
             "-j2"
           ],
           "fileset": "vm",
-          "shards": 15
+          "shards": 10
         }
       ]
     },