Updater picker empty list message to match media type

[jhnstn]

Fixes wordpress-mobile/gutenberg-mobile#3167

To test:
0. Choose a site with no media in the WordPress media library

1. Open up the editor and add one of each blocks to a post: Image, Video, Audio and Media
2. For each block try adding a file from the "WordPress Media Library"
3. Verify that the empty media message matches the block type i.e. Image block -> "You don't have any images", etc
4. Open the post on a device with no media ( such as an emulator )
5. For each block try adding a file via "Choose from device"
6. Verify that the empty media message matches the block type.

Regression Notes

1. Potential unintended areas of impact
   This touches how the media picker collects files and could impact non empty file systems

2. What I did to test those areas of impact (or what existing automated tests I relied on)
   I tried the picker on sites with and with out media

3. What automated tests I added (or what prevented me from doing so)
   None were added.

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

	Warnings
⚠️	PR is not assigned to a milestone.

Generated by 🚫 dangerJS

[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[peril-wordpress-mobile]

You can test the changes on this Pull Request by downloading the APKs:

• WordPress-pr-16050-build-120624.apk

• Jetpack-pr-16050-build-120624.apk

[mchowning]

Just a few small issues, but nothing major, so I'll go ahead and give a 👍 because this worked great in my testing. Thanks Jason! 🙇

I did notice that this is a slightly different approach to how this is handled in the MediaGallery Fragment, which uses MediaFilters. I think that's fine because I like how simple this implementation is, but I wanted to at least call it out since it's always nice to use the same approach when possible.

[mchowning]

Unless we want a null value for mediaTypes to be distinguishable/treated-differently from from an "empty set" of mediaTypes, I would be inclined to not allow this parameter to be null (i.e., to change it from Set<MediaType>? to Set<MediaType>) and to instead use emptySet() anywhere it would be null.

I just like to disallow null values if it makes sense. What you have now is fine though, and I think this falls in the realm of personal preference, so feel free to leave it as-is if you prefer.

[jhnstn]

I'm probably not as nullphobic as I should be but there might be a reason for this to be null. That might however be an artifact from when I was trying to figure out the new interface to hold the mediaTypes. I'm guessing I can switch this out to use an emptySet