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
Remove unneeded require statement in tests
Address code review feedback for system.storage
Merge pull request #411 from MobileChromeApps/system-storage

Implement chrome.system.storage for Android and iOS