releasetools: Add common.ZipDelete().

We have been shelling out to 'zip -d' to delete existing ZIP entries in
add_img_to_target_files.py. This CL moves the function into common.py,
and calls that for the similar work in ota_from_target_files.py. This CL
also adds unittests for the newly added function.

Test: `m dist`
Test: python -m unittest test_common
Test: ota_from_target_files.py generates identical packages w/ and w/o
      the CL (so we know the streaming property computation is intact).
Test: Run 'add_img_to_target_files.py -a' that triggers a call to
      ReplaceUpdatedFiles().
Change-Id: Icaa6c3ea3ee2166023f78fa79275295f837ea842