releasetools: Fix the broken validate_target_files.py.

The file was broken due to earlier touches:
 - Missing 'import zipfile';
 - Mismatching arguments when calling GetSparseImage().

Bug: 73996151
Test: Run validate_target_files.py with a walleye-target_files.zip.
Test: pylint --rcfile=pylintrc validate_target_files.py
Change-Id: I3692bd51fb27a3da698e06b75155e84502549f66