Increase number of 'Windows tool_integration_tests' subshards - #189292

…r#196243)

The ten `Windows tool_integration_tests shards` are balanced at 14.8
minutes of tests each as of flutter#195628, which with 4-5 minutes of bot setup
makes 18-20 minute builders. Fifteen shards bring the test step to about
10 minutes and the builder under 15 min.

Another attempt at flutter#189292 but
now they are balanced post- flutter#195628.

Dropped the denominator from the build name so we don't need to have the
`bringup` thrash just to add or remove shards. See
flutter#189040.

The old ten _N_10 targets will be removed in a follow-up once these are
green in post-submit.

Fixes flutter#189290

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