Media external_id handling not honored during migration

Problem/Motivation

After performing the media_acquiadam migration, external_id for all media is same.

Steps to reproduce

• Have a umami (or similar demo site) installed
• Install media_acquiadam, media_acquiadam_example & media_library
• Authenticate site in the configuration and then authorize the user
• Add some DAM assets to the local site, using Acquia DAM Image.
• Add media field with Acquia DAM Image bundle to page/article content type(create if content type not exist).
• Add some content of type page/article with media local and dam both.
• Checkout the ACMS 3570 branch. Ensure you have acquia_dam 1.1 as well.
• Enable acquia_dam module
• Visit the migration page: /admin/acquiadam/migration
• Perform the migration

Proposed resolution

Media external_id should be updated automatically after migration.

Remaining tasks

• Figure out solution.
• Create MR
• Request review
• RTBC
• Merge
• Release

User interface changes

No

API changes

No

Data model changes

No