Make merge_target_files more lenient, needed for cross-release merging.

These changes were necessary to begin merging a new cross-release target
whose vendor half is frozen.

- MergeDynamicPartitionInfoDicts
  - Filters combined fields to remove duplicates
  - Merges `super_block_devices` as well as other keys that were not
    previously used by other targets consuming this tool.
- Introduces --allow-duplicate-apkapex-keys. This gives a warning rather
  than fatal error on duplicate apk/apex keys. This flag is needed for
  targets that cannot update a frozen half.
- (Formats merge_target_files.py)

Bug: 170683837
Test: Use merge_target_files to merge an S+R build, and boot.
Change-Id: Id5f787e730de8f8ef697e1f2f29ac6514221e58d