releasetools: Add dict-like setter to BuildInfo.

BuildInfo already has dict-like getters. This CL adds the matching
setter method to keep it backward compatible with device-specific
scripts that expect a dict.

It also adds items() method that allows iterating the items.

Bug: 111087332
Test: python -m unittest test_ota_from_target_files.BuildInfoTest
Test: Generate an incremental OTA that sets info_dict in device-specific
      releasetools script.
Change-Id: Idd033f98a9186740f9da1a300d4c2bdddd9c1345