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

[ItsHarper]

I originally posted this here, but I'm reposting (with some more detail) since that repository has been archived.

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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 using a third party file manager, upload it to Standard Notes, and finally delete the copy in general storage. If I didn't have a file manager app installed that adds a "Save as" option to the Share menu, I'd have had to export my recording by using an insecure cloud service!

This may apply to iOS too.