releasetools: Make additional modules Python 3 compatible.

Bug: 131631303
Test: `python -m unittest test_sign_target_files_apks`
Test: `python3 -m unittest test_sign_target_files_apks`
Test: `python -m unittest test_add_img_to_target_files`
Test: `python3 -m unittest test_add_img_to_target_files`
Test: `python -m unittest test_ota_from_target_files`
Test: `python3 -m unittest test_ota_from_target_files`
Test: `python -m unittest test_validate_target_files`
Test: `python3 -m unittest test_validate_target_files`
Test: Run `python3 ota_from_target_files.py` to generate an OTA.
Test: Run `python3 sign_target_files_apks.py` to sign a target_files.
Change-Id: I56b45bbcbf7aa83e690785a9640c0212e45d12d8