Handle virtual A/B defines during dynamic_partitions_info merge.

While merging the Dynamic Partition info Dicts in the case of
split builds, pick the virtual-a/b flags like virtual_ab,
virtual_ab_retrofit from the vendor half.

Bug: 146811936
Test: Merged build contains the virtual a/b defines.
Change-Id: I0171b074094e47cfd12dc6af2565acdcd1b65163