set() doesn't keep elements according to the order of insertion. So
Transfers managed with set() in intermediate steps may not appear in the
same order across runs. This leads to slightly different output packages
when generating the same incremental OTA.
This CL fixes the issue by replacing set() with OrderedDict() in
blockimgdiff.GenerateDigraph() and blockimgdiff.FindVertexSequence().
It also adds a testcase that ensures blockimgdiff.GenerateDigraph()
preserves the insertion order for Transfer.goes_after set.
Bug: 32220816
Test: ota_from_target_files.py gives identical package when running
multiple times.
Change-Id: I56d551e5ca926993ab46896e33c80e0ce42e506a
3348228423