Review file access strategy

[valllllll2000]

Currently we target api 29 in both flow and caddisfly apps. We need to update to target api 30 soon as eventually new/existing apps will only be admitted on play store if they target 30. There was already an issue on Caddisfly https://github.com/akvo/akvo-caddisfly/issues/254 which was solved by using this temporary flag which has been also added to flow. Problematic areas:

• Caddisfly images
• Taking a video (error while deleting the copy)
• akvoflow/inbox folder -> loading zip file no longer works, we need to see how to handle those files from now on: the "open with" option or go to flow and manually "browse file".
• published files

The idea behind this change by Google is apps do not have access to file system but only to their dedicated app folders. Those folders cannot be browsed by other apps. Placing the form zip file in the public folder is the problematic one because we need to find a different way to handle that. There is still a possibility to have access to the whole external storage but we need to add instructions at the start of the app so users allow this permission (see https://developer.android.com/preview/privacy/storage#all-files-access). As much as possible for security reasons, I think we should avoid that solution as we don't really need access to the whole filesystem of the device.