fix: Reorder attribute merge to apply user attributes first

[Mansi-mParticle]

Instructions

1. PR target branch should be against development
2. PR title name should follow this format: https://github.com/mParticle/mparticle-workflows/blob/main/.github/workflows/pr-title-check.yml
3. PR branch prefix should follow this format: https://github.com/mParticle/mparticle-workflows/blob/main/.github/workflows/pr-branch-check-name.yml

Summary

• Previously, we were adding the selectPlacement attributes first and then applying the user attributes, which would override the initial selectPlacement attributes. This would indicate prioritizing user attributes over selectPlacements attributes.
• After this PR, we apply the user attributes first and then add the selectPlacements attributes, so any collisions will give the selectPlacement attributes priority.

Testing Plan

• Was this tested locally? If not, explain why.
• Tested with sample app.

Reference Issue (For mParticle employees only. Ignore if you are an outside contributor)

• Closes https://go.mparticle.com/work/REPLACEME