releasetools: Don't return ZipFile from common.UnzipTemp().

In addition to the unzipping work, common.UnzipTemp() kindly bundles an
open ZipFile object as part of the return value. It doesn't look very
helpful to the callers though. It also looks less obvious that the
caller needs to properly close the handle (missing the close here is
benign though). This CL just removes the ZipFile object out of the
return value, and leaves the work to callers.

Test: `m dist` on both of A/B and non-A/B target.
Test: python -m unittest test_add_img_to_target_files
Test: python -m unittest test_common
Test: python -m unittest test_ota_from_target_files
Test: Check the callers to common.UnzipTemp() in code search.
Change-Id: Id47da3fd42a0e76d6ae8851f05780db319ee48cf