releasetools: Add a BuildInfo class.

Prior to this CL, we have similar codes in different paths
(WriteFullOTAPackage / WriteBlockIncrementalOTAPackage /
WriteABOTAPackageWithBrilloScript). This CL factors out the common codes
that deal with OEM-specific properties into BuildInfo class, and adds
tests for the new class.

Test: python -m unittest test_ota_from_target_files
Test: Create an incremental package that uses and doesn't use OEM
      properties respectively.
Test: Create a full package that uses and doesn't use OEM properties
      respectively.
Change-Id: Ida914cace12803d55396fa503bafcac2db2a520e