releasetools: Create PropertyFiles class.

And move StreamingPropertyFiles as its subclass. We will need similar
PropertyFiles instance for non-A/B OTA as well (to expose the
offset/size for the METADATA entry).

Bug: 74210298
Test: python -m unittest test_ota_from_target_files
Test: Generate an A/B OTA. Check the generated property-files string.
Test: pylint --rcfile=pylintrc \
          ota_from_target_files.py \
          test_ota_from_target_files.py
Change-Id: If90d97f0b330749fd8a6cde2ed9d0d6cd6ea60a8