Maintain last-saved across form updates

[grzesiek2010]

Closes #5557

What has been done to verify that this works as intended?

I've tested the fix manually and added automated tests.

Why is this the best possible solution? Were any other approaches considered?

Copying the last-saved.xml file while downloading a form update is the most obvious solution here since it's similar to what we do with media files that already exist and are also used by the updated version of the form.

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

Any form that uses https://docs.getodk.org/form-logic/#values-from-the-last-saved-record
I've used:
LastSavedTest.xlsx

Do we need any specific form for testing your changes? If so, please attach one.

No.

Does this change require updates to documentation? If so, please file an issue here and include the link below.

No.

Before submitting this PR, please make sure you have:

• run ./gradlew checkAll and confirmed all checks still pass OR confirm CircleCI build passes and run ./gradlew connectedDebugAndroidTest locally.
• verified that any code or assets from external sources are properly credited in comments and/or in the about file.
• verified that any new UI elements use theme colors. UI Components Style guidelines

[dbemke]

Tested with Success!

Verified on device with Android 10

Verified cases:

• Maintain last-saved across form updates #5557
• last saved in updated forms (with version and the first without version)
• editing, finalizing and sending forms with last saved
• form update modes: manual, previously dowloaded forms only, match exactly
• hide old form versions enabled and disabled
• deleting and downloading form with last saved

[srujner]

Tested with Success!

Verified on device with Android 13