releasetools: Track the change to BuildInfo.

The global GetBuildProp() has been factored into BuildInfo class.

ota_from_target_files stderr: Traceback (most recent call last):
  File "/tmp/otatest1734786148718419730/releasetools/ota_from_target_files.py", line 1509, in <module>
    main(sys.argv[1:])
  File "/tmp/otatest1734786148718419730/releasetools/ota_from_target_files.py", line 1421, in main
    source_file=OPTIONS.incremental_source)
  File "/tmp/otatest1734786148718419730/releasetools/ota_from_target_files.py", line 1117, in WriteABOTAPackageWithBrilloScript
    max_timestamp = GetBuildProp("ro.build.date.utc", OPTIONS.source_info_dict)
NameError: global name 'GetBuildProp' is not defined

Test: Build an incremental A/B package with "--downgrade" flag.
Change-Id: I5c304c8b1ea79f3e5465b203bb990cbdffb9b328