releasetools: Move verity related functions into verity_utils.py.

This prepares for upcoming changes that refactor verity-related
functions into classes.

This CL makes minimal changes to the moved functions, by creating a new
BuildVerityImageError class in verity_utils.py replacing the former
BuildImageError that's specific to build_image.py. As part of the
change, it also moves the tests for AVBCalcMinPartitionSize into
test_verity_utils.py.

Test: python -m unittest test_verity_utils
Test: `m dist` with aosp_marlin-userdebug (Verified Boot 1.0)
Test: `m dist` with aosp_taimen-userdebug (Verified Boot 2.0)
Change-Id: I19b52714d8980705ea1f9484ac03eb0af9483240