Add host app implementation of NativeNotificationContentFetcher for development app:
- Create PluginApplication extends Application to register fetcher on app startup
- Create DemoNativeFetcher implementing NativeNotificationContentFetcher interface
- Register PluginApplication in AndroidManifest.xml
- DemoNativeFetcher returns mock notification content for testing
This demonstrates the SPI pattern where host apps provide their own
content fetching implementation to the plugin for background workers.
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