[New architecture] Get public shares for a file

[davigonz]

Get public shares for a file is the first use case to start implementing #2351 , following the next architecture:

Thanks to the ViewModel and LiveData , the shares shown in the view will be binded to the database, updating them automatically when a new change is detected.

---

TASKS

• Include needed dependencies: Kotlin, room, lifecycle, testing...
• Create Share entity
• Create ShareDAO with query to get public shares for a file
• Use LiveData in ShareDAO
• Implement Room database
• Create ShareRepository
• Create ShareViewModel
• Connect SharesFragment with ViewModel to retrieve automatically the public shares.
• Observe shares and errors with livedata
• Error handling
• Expose shares stored in room by using Content Provider.
• Add additional state (LOADING) to show loading dialog.
• Database migration
• Tests:
  • ShareContentProvider
  • OCLocalDataSource
  • OCShareDAO
  • OCRemoteDataSource
  • OCShareRepository
  • NetworkBoundResource
  • OCShareViewModel
  • UI tests