releasetools: Prefer the avbtool specified in target_files.

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