Sort AB_OTA_PARTITIONS

Some product config makefiles are being refactored for
the conversion to Starlark. In order to test that the
refactor doesn't cause any meaningful change, we test
that the ninja files are the same before/after the refactor.
Currently, they have differences in the order of AB_OTA_PARTIONS,
which shouldn't matter. In order to increase our confidence
and make the ninja files more stable, sort AB_OTA_PARTITIONS
before it's used.

Bug: 220169197
Test: Presubmits
Change-Id: I6a9f9f2990406d810577cf8930416925f2987311