Clean up merge_target_files

Summary of changes:
- Rename extract_items args to be more generic.
- For many other functions, replace large argument lists with
  the OPTIONS shared object.
- Rename process_* functions to merge_* to be more clear about
  the purpose of the function.
- Remove verbose code in merge_ab_partitions_txt().
- Extract certain logic from merge_misc_info_txt() and place it
  into other existing functions: dynamic partition and selinux
  keys are now processed in their own functions.
- Remove verbose code in merge_package_keys_txt().
- Remove verbose code in create_file_contexts_copies().
- Handle extracting from META directly in merge_meta_files(),
  rather than doing this extraction in create_merged_package()
- Storing the set of framework and vendor partitions in OPTIONS
  so it can be reused by multiple functions without needing to
  be parsed again.

Bug: 221858722
Test: atest test_merge_target_files
Test: Use merge_target_files to merge some builds
Change-Id: I8aa429c8fbb05223127b116aab84b2678ac264a8