releasetools: Clean up three functions in add_img_to_target_files.py.

This CL makes the following changes:
 (a) cleans up the similar codes in AddCareMapTxtForAbOta() that handle
     'system' and 'vendor' partitions;
 (b) fixes an issue with the arcname in AddPackRadioImages() and
     AddRadioImagesForAbOta(), where forward slash should always be used
     in zip entry names;
 (c) refactors the branching statements in AddRadioImagesForAbOta() to
     reduce indentation levels.

Test: python -m unittest test_add_img_to_target_files
Test: `m dist` with aosp_marlin-userdebug. Check META/care_map.txt in the
      generated target_files.zip.
Change-Id: I3d6f794962d0c68390fbd18eb13c2622acab3ff5