Switch Dawn Nexus 5X testing to O

Switches the Dawn Android builders to trigger tests on Nexus 5Xs with
O instead of N since N has been deprecated.

Submitted with no-try since CL submission is blocked otherwise due to
being unable to run tests on N anymore. The only "testing" we do on
these devices right now is a no-op suite, though, so there shouldn't
be any risk of things breaking. Nexus 5xs with O are already used for
other tests using the same harness.

No-Try: true
Bug: 1498625
Change-Id: I733432d43986f6526090c5f2b58923266abab4be
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4995042
Auto-Submit: Brian Sheedy <bsheedy@chromium.org>
Commit-Queue: Austin Eng <enga@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1218486}