[SPACES] Document Provider showing available Spaces

[jesmrec]

Document provider feature will show the available spaces for an specific account.

By selecting an account in document provider, a list of spaces is displayed, every of them as a folder. Then, user will browse through it as usual.

No changes for non-spaces accounts.

Article with info about DocumentsProvider: https://medium.com/androiddevelopers/building-a-documentsprovider-f7f2fb38e86a

TASKS

• Research
• Create branch spaces/document_provider
• Development tasks
  • Create new SpaceCursor to show spaces
  • Modify logic in RootCursor to set correct directory to COLUMN_DOCUMENT_ID depending on whether it's a spaces or non-spaces account
  • Adapt logic in DocumentsStorageProvider to show the list of spaces
  • Adapt logic to be able to browse through the hierarchy of spaces as if they were folders
  • Check that create folder, rename and remove operations work properly
• Code review and apply changes requested
• Design test plan
• QA
• Merge branch spaces/document_provider into spaces/main