releasetools: Unzip all files when generating image archive.

Commit 7df64c3e starts to call common.LoadInfoDict() when generating
image archive, which reads additional files under BOOT/, RECOVERY/ and
ROOT/. Unzip everything from the target_files.zip.

Bug: 132456827
Test: Run img_from_target_files.py on previously failing
      target_files.zip.
Change-Id: I22ee57c4f765bee9494478bf115b1581877401f4