APV: Skip AU header signature bytes when forming CSD

New APV spec according to OpenAPV 0.1.11.3.1 adds extra 4 bytes to AU
header. We need to skip those when creating CSD in the encoder

Bug: b/406782106
Bug: b/406819921

Flag: EXEMPT bugfix

Test: MctsMediaV2TestCases on raven and panther
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:d96b39dd6cb8ac52da38a51498ecc613d9b0843b)
Merged-In: I50d94b24512b5d584fa20fc4e516561a54f59d02
Change-Id: I50d94b24512b5d584fa20fc4e516561a54f59d02