[bots] Increase number of shards for vm-aot-android-* bots

Shards on vm-aot-android-debug-arm64c and vm-aot-android-debug-arm_x64
bots have been timing out recently, exceeding 1.5h testing time.
It seems like they are making progress, but number of tests is quite
large.

Increase number of shards from 10 to 16 in order to reduce running
time of each shard and avoid timeouts.

Change-Id: Id53d2cdf1eaa2606874118153c0f9c822c212daf
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/461840
Auto-Submit: Alexander Markov <alexmarkov@google.com>
Reviewed-by: Alexander Thomas <athom@google.com>
Commit-Queue: Alexander Markov <alexmarkov@google.com>

Author: alexmarkov

tools/bots/test_matrix.json

@@ -823,7 +823,7 @@
             "-j2"
           ],
           "fileset": "vm",
-          "shards": 10
+          "shards": 16
         }
       ]
     },