releasetools: Separate streaming metadata computation into functions.

And add unittests for ComputeStreamingMetadata().

This prepares for the changes that add additional property-files (for
both of A/B and non-A/B).

Bug: 74210298
Bug: 72751683
Test: python -m unittest test_ota_from_target_files
Test: Generate A/B OTA package. Check the ota-streaming-property-files
      in the METADATA entry.
Change-Id: Ib4b069f61c2c06c035c0cff73a55112f3936b969