Merges dynamic partition misc_info.txt keys from both system and other.

To build a complete list of the dynamic partitions and partitions
groups, we need to merge the contribution from the system and other
target files.

Bug: 127687287
Test: Running merge_target_files.py and observing partition lists are
merged as expected.
Change-Id: I5bb9bd0e3179d48c9bfacdb3aca8253158f61cf6