releasetools: Refactor AddImagesToTargetFiles().

Separate three functions out of AddImagesToTargetFiles(), into
AddCareMapTxtForAbOta(), AddRadioImagesForAbOta() and
AddPackRadioImages() respectively. This CL tries to apply minimal
changes for the refactoring purpose.

Also add tests for AddRadioImagesForAbOta() and AddPackRadioImages().
The tests for AddCareMapTxtForAbOta() require better testing support to
mock sparse_img.SparseImage, which will be added in later CLs.

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: I1bb723c15237ff721f165cfce0ce996008ce9948