Fix polling interval settings and add release build signing

- Fix polling interval not reflecting saved value (use SharingStarted.Eagerly)
- Add release build signing configuration with keystore
- Add key.properties to .gitignore for security
- Add ProGuard rules for JSpecify annotations

.gitignore

@@ -19,6 +19,9 @@ captures/
 *.jks
 *.keystore
 
+# Key properties (contains sensitive signing info)
+app/key.properties
+
 # External native build folder generated in Android Studio 2.2 and later
 .externalNativeBuild
 .cxx/