Ensure insertion order of Python dict

Add keys to the dict `partitions` in serial to have a deterministic
insertion order.
This guarantees the generated vbmeta.img is always the same on the same
input, fixing image consistency issues.

Reference:
https://android-review.googlesource.com/c/platform/build/+/2585397

Bug: 332550989
Test: Generate vbmeta several times, confirm identical result.
Change-Id: I39da5844045f497fdc89e6477a7e32ddacb70764