fix: locate apk in agp 9 output dir

[guzino]

Refs synonymdev/bitkit-android#1128
Refs synonymdev/bitkit-android#1127

This PR points the Android build helper at the APK directory produced by the AGP 9 build.

Description

AGP 9 removes the legacy variant APIs that renamed APKs inside AGP's own output directories, so synonymdev/bitkit-android#1127 leaves those outputs under their default app-* names and writes the established bitkit-* names as copies under app/build/outputs/bitkit/<variant>. The helper's lookup in app/build/outputs/apk/<flavor>/debug/ returns no file against that build and exits before staging anything.

• Replaces APK_FLAVOR_DIR with APK_VARIANT_DIR, using the Gradle variant names devDebug and mainnetDebug.
• Reads the universal APK from app/build/outputs/bitkit/$APK_VARIANT_DIR.
• Keeps the bitkit-*-universal.apk glob and the staged aut/bitkit_e2e.apk and aut/bitkit_e2e_mainnet.apk filenames unchanged, so no downstream E2E configuration moves.

QA Notes

Manual Tests

• 1. ./scripts/build-android-apk.sh against chore: upgrade to agp 9 and configure parallelism bitkit-android#1127 → stages aut/bitkit_e2e.apk from bitkit-dev-debug-187-universal.apk.
• 2. BACKEND=regtest ./scripts/build-android-apk.sh → stages aut/bitkit_e2e.apk from the same universal APK.
• 3. BACKEND=mainnet ./scripts/build-android-apk.sh → stages aut/bitkit_e2e_mainnet.apk. I could not run this: the checked-in google-services.json has no to.bitkit client, so the mainnet build stops before the lookup. The mainnet branch changes the same single path.
• 4. regression: full E2E run against the AGP 9 branch → suites resolve the staged APK as before.

Automated Checks

• bash -n passes and no APK_FLAVOR_DIR or outputs/apk reference remains in the script.
• Confirmed the previous lookup returns no file against the AGP 9 build, which is the failure this fixes.