This allows a consistent logic in using the avbtool which could be
board-specific.
Test: `atest releasetools_test`
Test: Run sign_target_files_apks.py on a target_files.zip.
Change-Id: I8cd93b8e71146985734f85c31f4662f5e2e9534c
This ensures a matching interface between sign_apex and apex_utils.
The test apex `testdata/foo.apex` is generated by running
`system/apex/apexer/runtests.sh`.
Test: python -m unittest test_sign_apex
Test: atest releasetools_test
Change-Id: I7c14b1df2a3038ad206aa3e5aac084c47baaa00b