Emulate IS_PENDING flag from API 21+

[yrezgui]

The IS_PENDING flag allows a file to not be visible to others applications until it has been processed like when you're still saving its content (see documentation).

The flag is only available from API 29+ but could possible be emulated on previous Android versions by following this scenario:

• Save the file content first in the app cache directory
• Move the file to the right MediaStore collection folder (see getExternalStoragePublicDirectory)
  • The operation should be quick as the file is already on the same storage volume
• Scan the file in MediaStore

[yrezgui]

We should think of possibilities with the new storage but closing this issue as it's related to the deprecated mediastore module