emu_downloads: add ANDROID_REPOSITORY env var to override download base URL

[jpcottin]

Why

All system-image and emulator-zip downloads are currently hardcoded to https://dl.google.com/.... That breaks for users behind corporate proxies, air-gapped environments, or in regions where Google's CDN is slow/unreachable — most prominently the long-standing #240 ("China available mirror image"). It also makes it hard to test new emulator releases via a staged mirror without forking the script.

Change

Two commits:

1. Add override of download location. (cherry-pick of @gregersn's commit from Custom naming and repository #359, authored 2021-10-11) — introduces an ANDROID_REPOSITORY env var, defaulted to https://dl.google.com, and uses it as the base for system-image catalogs (sys-img2-5.xml), the emulator repo catalog (repository2-1.xml), the per-package system-image zip URLs, and the platform-tools download.

2. normalize ANDROID_REPOSITORY trailing slash (mine) — small follow-up. The two modules disagreed on whether the default had a trailing slash (https://dl.google.com/ vs https://dl.google.com), and combined with format strings starting /android/... this produced https://dl.google.com//android/... (double slash). Normalized both defaults to no trailing slash and added .rstrip("/") so user-supplied bases with a trailing slash also work cleanly.

Conflict resolution notes

The original cherry-pick had two conflicts against current master, both resolved to keep HEAD's logic and apply the substitution:

• SYSIMG_REPOS: kept HEAD's newer sys-img2-5.xml filenames (Greger's commit predated the bump and still referenced sys-img2-1.xml), wrapped each in %s/... with ANDROID_REPOSITORY.
• AndroidReleaseZip.url construction: kept the url_dir = sort_base or self.tag logic (added later for 16KB-page sysimg URL routing — see the is_16k work), substituted ANDROID_REPOSITORY for the hardcoded base.

Credit

@gregersn opened the original feature in #359 (along with an unrelated --name flag). Splitting into two PRs (#410 for --name, this one for ANDROID_REPOSITORY) for reviewability. Original authorship preserved on commit 1 via cherry-pick.

Cross-link

Addresses #240 ("Requesting/Begging for a China available mirror image"). This PR provides the client-side override needed to point at any mirror; standing up the actual mirror is out of scope.

Test plan

• All 42 existing unit tests pass (pytest tests/ --ignore=tests/e2e)
• Functional verification of URL construction in three scenarios:
  • No env var: https://dl.google.com/android/repository/sys-img/android/sys-img2-5.xml
  • ANDROID_REPOSITORY=https://my-mirror.example.com: https://my-mirror.example.com/android/repository/sys-img/android/sys-img2-5.xml
  • ANDROID_REPOSITORY=https://my-mirror.example.com/ (trailing slash): same as above — rstrip prevents double-slash
• CI from ci: add GitHub Actions workflow to run pytest on PRs #405 reruns the matrix (Python 3.10/3.11/3.12/3.13)

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