Wire up the real 2GIS MapKit SDK (was a placeholder)

Added the actual ru.dgis.sdk:sdk-map + compose-map dependencies (from
https://artifactory.2gis.dev/sdk-maven-release) instead of the Canvas
placeholder guess. Verified the exact API surface by downloading the
AARs directly and inspecting classes with javap (DGis.initialize,
MapOptions, MapComposableState/MapComposable, CameraPosition/GeoPoint)
rather than relying on possibly-stale docs.

DgisSdkProvider lazily calls DGis.initialize() once per process,
catching failure so a missing key doesn't crash the app; DgisMapView
renders the real MapComposable when that succeeds, falling back to
the old placeholder canvas otherwise.

Important finding: the existing DGIS_API_KEY (2GIS's public REST/JS
API key format) does NOT work with this native SDK. It requires a
separate dgissdk.key file issued per-app-package from dev.2gis.com,
placed in app/src/main/assets/ (gitignored). Documented in README.
Without it the app still runs fine on the placeholder map.

Also restricted ndk.abiFilters to arm64-v8a — the SDK's native libs
otherwise balloon the APK from ~19MB to ~190MB.

Verified: testDebugUnitTest passes, assembleDebug produces a working
(arm64-only, ~69MB) APK. Not runtime-verified against a real map key
or device — no emulator/device available in this environment.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.gitignore

@@ -27,6 +27,7 @@ android/captures/
 android/.externalNativeBuild/
 *.keystore
 *.jks
+android/app/src/main/assets/dgissdk.key
 
 # IDE
 .vscode/