Promote GPU builder "Android Release (Pixel 2)" to CQ

The change includes:
1. Enable "Android Release (Pixel 2)" on CQ and branches.
2. Disable "Android Release (Nexus 5X)" on CQ and branches.

The average test phrase is 8 mins on Pixel 2 vs 12 mins on Nexus 5X:
https://screenshot.googleplex.com/9V2J5J6d9EaJieR so there shouldn't
be a regression on CQ build cycle time.

Graph http://shortn/_7DDeoN8D89 shows the peak usage of Nexus 5X on
Oreo is ~200. Graph http://shortn/_0HjkCIbCjN shows the available
Pixel 2 on Pie is ~200 so they should be able support CQ traffic.

Bug: 355671260
Change-Id: I732249d72b847f9c722fc967b366c3cd355d2755
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5754565
Commit-Queue: Haiyang Pan <hypan@google.com>
Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Reviewed-by: Erik Staab <estaab@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1336284}