[FEATURE REQ] Clean up the "Repository Contents" section of the README

The Repository Contents sections of the README is confusing. To an android developer like myself, it's not clear which sample I should consult in order to use the library in my app.

There's:
1. [notification-hubs-sample-app-java](https://github.com/Azure/azure-notificationhubs-android/blob/main/notification-hubs-sample-app-java)
2. [notification-hubs-sample-app-legacy](https://github.com/Azure/azure-notificationhubs-android/blob/main/notification-hubs-sample-app-legacy)
3. [FCMTutorialApp](https://github.com/Azure/azure-notificationhubs-android/blob/main/FCMTutorialApp)

For a new user of the library, the relevant one seems to be the last one (**FCMTutorialApp**). Whichever it is, it should be the first one in _Repository Contents_ sections.

The rest should be below that one, preferably in a section with a lower heading.

----

Example:

# Repository Contents

[FCMTutorialApp](https://github.com/Azure/azure-notificationhubs-android/blob/main/FCMTutorialApp)
[...]

## Legacy
[notification-hubs-sample-app-java](https://github.com/Azure/azure-notificationhubs-android/blob/main/notification-hubs-sample-app-java)
[...]

[notification-hubs-sample-app-legacy](https://github.com/Azure/azure-notificationhubs-android/blob/main/notification-hubs-sample-app-legacy)
[...]

## Other
[com.microsoft.windowsazure.messaging](https://github.com/Azure/azure-notificationhubs-android/blob/main/notification-hubs-sdk/src/main/java/com/microsoft/windowsazure/messaging)
[...]

[notification-hubs-sdk-e2etestapp](https://github.com/Azure/azure-notificationhubs-android/blob/main/notification-hubs-sdk-e2etestapp)
[...]

Start with the Repository Contents section of the README and compare the listed sample links with the requested grouping. Put FCMTutorialApp first, move the other samples under a lower Legacy heading, and place the SDK and end-to-end app under Other; done when the README clearly directs new Android users to the relevant sample.

Written by the indexing model from the issue text.