Add a fallback for user displayName when this one is null or empty

[yostyle]

Actually if user displayName is null or empty the function getBestName() return the matrixId.
It should be better to have a fallback as for the room name :RoomDisplayNameFallbackProvider

For example we have this fallback on Tchap : https://github.com/tchapgouv/tchap-android-v2/blob/27fd9730546e7f51734868e61433cb5a4bf4cafd/vector/src/main/java/fr/gouv/tchap/core/utils/TchapUtils.kt#L89

[bmarty]

The current plan is:

• delete all existing getBestName() method
• SDK side, create a DisplayNameService, with one method getBestName(matrixItem)
• On MatrixConfiguration, add an interface to let the app provide an optional userDisplayNameFallbackProvider. If null, default fallback will still be the matrixId
• The SDK implement DisplayNameService in DefaultDisplayNameService, injecting the MatrixConfiguration.
• DisplayNameService is available from the session

[bmarty]

In EA 1.3.2