releasetools: Update the embedded AVB pubkey in an APEX.

APEX keys are now embedded in the APEX [1].

[1] https://android-review.googlesource.com/c/platform/system/apex/+/936942

Fixes: 128297564
Test: Run sign_target_files_apks.py on a target_files.zip built with the
      CL above.
Change-Id: I219cc3e6ce6f652537fcc37322c8902f295c1447