releasetools: Support signing APEXes.

Bug: 123716522
Test: Run sign_target_files_apks.py to sign a target_files with APEXes.
Test: Run check_target_files_signatures.py on signed artifact.
Test: python -m unittest test_sign_target_files_apks
Change-Id: I3fa13e3d9461cf5e0838e0572d436e218164fe41
(cherry picked from commit aa7e993a)