Basically, the crash was caused by using `Collections.unmodifiableList()`, which creates an `UnmodifiableCollection` wrapper around the original `List`.
That `UnmodifiableCollection` wrapper contains methods that return `java.util.stream.Stream` – which was added in Java 8, but can be used on Android API < 24 through [desugaring](https://developer.android.com/studio/write/java8-support-table).
_However_, it appears that when Jackson is using reflection to serialize `ReserveUsernameRequest`, it examines all of the methods of that class. This results in a `java.lang.NoClassDefFoundError: java.util.stream.Stream`, because the reflection is looking at the actual runtime `UnmodifiableCollection` class and not desugared code.
As far as I can tell, that behavior is unavoidable and the only way to avoid this issue is to avoid using `Collections.unmodifiableList()` in classes that are serialized by Jackson on Android API < 24.
`AttachmentSaver` was missing logic to show a toast message after the user denies `WRITE_EXTERNAL_STORAGE` permission.
#### Changeset
- Add missing toast after write external storage permission is denied.
- Add unit test coverage for `AttachmentSaver` result messages.
- Rename `AttachmentSaver` string resource names so they all have the same prefix.
Story playback was previously paused when the context menu is opened, but resumed while saving the media.
With this change, playback will remain paused while saving media, so the user doesn't potentially miss any stories while clicking through the dialogs to save media to their device storage.
Introduces `AttachmentSaver` to centralize all of the steps needed to save message attachments to the device storage. It handles the entire workflow including:
- Showing the save to storage warning/confirmation dialog.
- Requesting `WRITE_EXTERNAL_STORAGE` permission.
- Showing/dismissing media save progress.
Goals of this new class:
- Make it easy to save media attachments anywhere with just a few lines of code (and easier to replace the deprecated `SaveAttachmentTask`).
- Ensure all of the necessary steps are consistently performed at each usage site (which wasn't the case before).
- Make it easier to unit test the save attachment logic.
