Add BuildInfo::set_gms_version_code_for_test() (part 1/3)

This is part of a three-sided patch to allow overriding the GMS
version code in tests.
https://crrev.com/c/5752470 (1/3)
https://crrev.com/i/7545924 (2/3)
https://crrev.com/c/5752697 (3/3)

This part exposes a getGmsVersionCode() method on top of the public
gmsVersionCode member, and migrates uses of the member to the
method.

Bug: 346556567
Change-Id: Ia6fc09d3fb98c6adabd6c63696270bc2198fde77
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5752470
Commit-Queue: Victor Vianna <victorvianna@google.com>
Owners-Override: Andrew Grieve <agrieve@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1335695}