releasetools: Fix alignment issue when signing APEXes.

Previously it was following a wrong order by doing `zipalign` after
calling SignApk, which effectively compromised the signature. This CL
corrects the logic, and follows the same flow as in build system:
 - Pack APEX file;
 - `zipalign -f 4096`;
 - Call SignApk to sign the container with `-a 4096` flag.

Bug: 129148142
Test: Run sign_target_files_apks.py on taimen target_files.zip. Boot the
      image after signing.
Change-Id: I91bd3dce4f45c1891c5e122212a699f4808618fa
(cherry picked from commit 0e06cb0a)