Merge identical key/val pairs in dynamic partition info

We might add new values to dynamic_partition_info.txt, so some kind of
generic fallback mechanism is needed. If keys are different, we need to
decided on a case-by-case basis which side takes precedence. For
example, Virtual AB requires vendor support, so vendor side takes
precedence. VABC on T+ devices are implemented entirely in system, so
system/framework side takes precedence.

Bug: 230876542
Test: th
Change-Id: I67747368547d3ef3e29ad64f8f818ef4c5896246