Replace 2GIS MapKit with OpenStreetMap (osmdroid) — no key needed

Getting a working mobile-SDK key from 2GIS turned out to require a
sales-mediated B2B process (Platform Manager subscription, demo keys
explicitly excluded from mobile SDK use per their own docs), not a
quick self-serve signup. Swapped to osmdroid instead: free, no API
key or account, works immediately.

CityMapView (replacing DgisMapView/DgisSdkProvider) wraps osmdroid's
View-based MapView via AndroidView, forwarding lifecycle events, with
markers for the user's location and each nearby place. Configured
required OSM tile-usage-policy user agent + app-private tile cache in
GuideCityApp. Removed the now-unused DGIS_API_KEY plumbing and the
arm64-only ABI filter (osmdroid has no heavy native libs, so it's not
needed) — APK is back down to ~19.6MB from ~69MB.

Verified: testDebugUnitTest passes, assembleDebug produces a working
APK, sent via the Telegram bot (previously blocked by its 50MB limit
with the 2GIS build).

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

.gitignore

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