Don't generate hashtree when signing bundled APEXes.

Bug: 139957269
Test: Sign a target_files.zip. Extract a re-signed APEX and check the
      hashtree size (being zero).
Test: Use sign_apex to sign an APEX file. Check the hashtree size (not
      being zero).
Test: python -m unittest test_apex_utils
Test: python -m unittest test_sign_apex
Change-Id: I927b7681d66920d7732b700ec3a8f7a65b4cb351