Crashlytics: use R8 r8-map-id on AGP 8.12+

[jrodiz]

#6770

injectCrashlyticsMappingFileIdRelease minted a new random id on every minified release build and wrote it into a generated resource, cascading through mergeResources → R8 → packaging and invalidating the build cache on every release.

Since AGP 8.12, R8 stamps a stable, content-derived map id (pg_map_id) into the mapping file and embeds the matching r8-map-id in obfuscated stack frames. On AGP 8.12+ we consume that id instead of minting our own — stable when the obfuscated output is unchanged, regenerated only when it changes. No churning id, no cache busting.

Changes

• New UploadMappingFileForR8MapIdTask task: reads the map id from the merged mapping file and uploads with it. Falls back to pg_map_id (the only id R8 emits today) but prefers an r8_map_id header defensively if a future R8 ever writes one. Warns and skips when no id is present.
• InjectMappingFileIdTask serves both paths via two factory methods: registerForR8MapId injects the constant USE_R8_MAP_ID sentinel (all ones, blank when no mapping is uploaded); register keeps the legacy blank/random behavior. Being constant, the AGP 8.12+ resource is byte-identical across rebuilds and keeps the release build cache valid.
• CrashlyticsPlugin gates on AGP version: >= 8.12 registers the sentinel inject + new upload tasks; < 8.12 keeps the original inject + upload behavior unchanged.

Verification

To confirm the standalone results reflect real AGP-driven builds, a minimal Android app (one kept entry class + one obfuscated helper, isMinifyEnabled = true, default optimize rules) was built with :assembleRelease across five AGP versions on JDK 17, each AGP paired with its required Gradle version, and the resulting app/build/outputs/mapping/release/mapping.txt header inspected:

AGP	bundled R8	compileSdk	pg_map_id length	r8_map_id present?
8.1.4	8.1.68	34	7 (prefix)	no
8.7.3	8.7.18	34	7 (prefix)	no
8.11.2	8.11.26	35	7 (prefix)	no
8.12.0	8.12.14	35	64 (full hash)	no
8.13.0	8.13.6	35	64 (full hash)	no
9.1.0	9.1.31	36	64 (full hash)	no
9.2.1	9.2.14	36	64 (full hash)	no

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