Implement chrome.system.storage for Android and iOS

- iOS: implement getInfo(), ejectDevice(), getAvailableCapacity()
- iOS: implement no-op support for onAttached/onDetached events
- Android: implement ejectDevice(), getAvailableCapacity()
- Android: implement onAttached/onDetached events for SD cards
- Update documentation to capture limitations of plugin
You should make explicit mention that on Android these events will wake up your app even if it's not running.