releasetools: Create StreamingPropertyFiles class.

This CL breaks down ComputeStreamingMetadata() into mutiple member
functions of StreamingPropertyFiles class, which correspond to the
two-pass logic when generating streaming property files (aka streaming
metadata).

StreamingPropertyFiles.Compute() does the work for the first pass, by
putting placeholders before doing initial signing. Finalize()
corresponds to the second pass, where the placeholders get replaced with
actual data. Verify() can be optionally called to assert the correctness
of the work.

The separation between Compute() and Finalize() is to allow having
multiple StreamingPropertyFiles instances (in coming up CLs). This way
we can call Compute() multiple times for each instance, followed by only
one call to SignOutput(). And similarly for Finalize().

Bug: 74210298
Test: Generate an A/B OTA package. Check the METADATA entry.
Test: python -m unittest test_ota_from_target_files
Change-Id: I45be0372a4863c4405e6d8e20bcb9ccdc29e7e11