Fix #6186: Flaky proguard in CI

[ShankhanilSaha]

Explanation

Fixes #6186

• In build_tests.yml & stats.yml, splits builds into separate jobs and shares Bazel cache via GH artifacts so final AAB packaging runs on fresh workers, avoiding Proguard OOMs.

Essential Checklist

• The PR title starts with "Fix #bugnum: " (If this PR fixes part of an issue, prefix the title with "Fix part of #bugnum: ...".)
• The explanation section above starts with "Fixes #bugnum: " (If this PR fixes part of an issue, use instead: "Fixes part of #bugnum: ...".)
• Any changes to scripts/assets files have their rationale included in the PR explanation.
• The PR follows the style guide.
• The PR does not contain any unnecessary code changes from Android Studio (reference).
• The PR is made from a branch that's not called "develop" and is up-to-date with "develop".
• The PR is assigned to the appropriate reviewers (reference).

[github-actions]

Coverage Report

Results

Coverage Analysis: SKIP ⏭️

This PR did not introduce any changes to Kotlin source or test files.

To learn more, visit the Oppia Android Code Coverage wiki page

[github-actions]

Coverage Report

Results

Number of files assessed: 6
Overall Coverage: 100.00%
Coverage Analysis: PASS ✅

Passing coverage

Files with passing code coverage

File	Coverage	Lines Hit	Status	Min Required
CurrentUserProfileIdIntentDecorator.kt utility/src/main/java/org/oppia/android/util/profile/CurrentUserProfileIdIntentDecorator.kt	100.00%	12 / 12	✅	70%
ProfileIdMigrationUtil.kt utility/src/main/java/org/oppia/android/util/profile/ProfileIdMigrationUtil.kt	100.00%	3 / 3	✅	70%

Exempted coverage

Files exempted from coverage

File	Exemption Reason
SplashActivity.kt app/src/main/java/org/oppia/android/app/splash/SplashActivity.kt	This file is incompatible with code coverage tooling; skipping coverage check.
PinPasswordActivity.kt app/src/main/java/org/oppia/android/app/profile/PinPasswordActivity.kt	This file is incompatible with code coverage tooling; skipping coverage check.
AudioLanguageActivityPresenter.kt app/src/main/java/org/oppia/android/app/options/AudioLanguageActivityPresenter.kt	This file is exempted from having a test file; skipping coverage check.
AudioLanguageActivity.kt app/src/main/java/org/oppia/android/app/options/AudioLanguageActivity.kt	This file is incompatible with code coverage tooling; skipping coverage check.

Refer test_file_exemptions.textproto for the comprehensive list of file exemptions and their required coverage percentages.

To learn more, visit the Oppia Android Code Coverage wiki page