Begins merging META/dynamic_partitions_info.txt.

This file is used by OTA generation so it needs to appear in mixed
builds with the combined content from the system and other versions of
the file.

Test: python -m unittest test_merge_target_files
Test: Running merge_target_files on a dynamic-partition-enabled build
and observing the resulting target files.
Bug: 131889742

Change-Id: I4ddbebc087e430f6307d0bd5461121a374e58ea4