Allow uploading files from other apps on Android via the Share mechanism

[ItsHarper]

For an optimal user experience, Standard Notes should expose an entry on the standard Android share screen, which allows the user to attach a file to a note.

For an example use case, if I use the Pixel Recordings app to make a recording, I should be able to share the audio file and transcript directly to Standard Notes for upload, rather than having to first export the recording to general device storage, and then manually upload it within Standard Notes, and then delete the copy on general storage.

This may apply to iOS too.