perf: Reduce android-pixel6-perf-pgo speedometer3

Reducing from effectively 15 to 5 (limited to 15 by number of bots).

This will improve the bot's passing rate as android-pixel6-perf-pgo is
one of the bots we are monitoring now for PGO. As much as running
speedometer more means better averaged results, we want the bot to pass
at least once every 3 days. Currently speedometer3 is one of the most
flaky perf tests due to the fact that it is run so many times. Reducing
this should improve the bot's pass rate.

Ideally this can be resolved by making "pass" equivalent to at least 90%
of the shards passing. But I don't know if this is possible with the
current infra.

Bug: 349387339
Change-Id: I0c52e97ed8b954123aaadaa5f051f6fcccdeebc9
Fixed: 349387339
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5653814
Reviewed-by: John Chen <johnchen@chromium.org>
Auto-Submit: Peter Wen <wnwen@chromium.org>
Commit-Queue: Peter Wen <wnwen@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1319413}