ci: cache emulator AVD for faster E2E boots

[ovitrif]

This PR adds AVD (Android Virtual Device) caching to E2E workflows to enable emulator "warm boots" instead of cold boots, reducing CI run times.

Description

• Add actions/cache@v4 step to cache ~/.android/avd/* and ~/.android/adb* directories
• Add snapshot generation step that runs only on cache miss to create the initial AVD snapshot
• Change emulator option from -no-snapshot to -no-snapshot-save to load cached snapshots without overwriting them
• Applied to both e2e.yml (main E2E tests) and e2e_migration.yml (migration tests) workflows

Expected impact:

• Cold boot: ~60-90 seconds
• Warm boot with cached snapshot: ~15-30 seconds
• Savings: ~45-60 seconds per emulator start
• With 6 shards, this could save 4-6+ minutes per E2E run on cache hits

Preview

N/A - CI workflow changes only

QA Notes

1. Trigger E2E workflow manually on this branch
2. Check logs for "AVD cache" step - should show cache miss on first run
3. Verify "Create AVD and generate snapshot for caching" step executes
4. Trigger E2E workflow again
5. Verify "AVD cache" step shows cache hit
6. Compare emulator boot times between runs
7. Verify E2E tests still pass with the new configuration

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

[ovitrif]

No idea if this actually does add a perf boost or it's detrimental to the speed instead, need to investigate with Claude.

[piotr-iohk]

ok