Android: Improved support for background launch in Android and iOS

[avargaskun]

Changes related to background launch:

• [iOS] Add documentation detailing how to configure the app to support launch from background using applinks
• [Android and iOS] Remember the NFC message/intent that caused the OS to start the application so it can be queried later during the deviceready or onresume application events.
• [iOS] If the app was already in the foreground when the tag was scanned, and the app registered an ndefListener, pass the NDEF messages to the listener directly

Additional bug fixes

• [Android] Fix a bug whereas writing a tag fails because savedIntent is not set on the reader callback during an NFC reader session.

Caveats:

• [iOS] The information available during background launch does not include tag metadata, only the NDEF message.

[Landlordway]

#471

[dwettstein]

Hi @avargaskun, thanks for your PR which basically fixes almost every problem in this repository. 👍
The only problem we still observe is scanning a tag when the app is in background or closed. With the PR #470 this works reliably.
Could you please have a look at it and merge this PR or parts of it as well?