ApexCodecs: move ApexCodec_Buffer to ApexCodecsImpl.h

In addition, misc fixes for param alignment & BAD_INDEX handling

Bug: 401332082
Bug: 435023366
Flag: android.media.swcodec.flags.apexcodecs_base
Test: MctsMediaV2TestCases MctsMediaDecoderTestCases
Change-Id: Ia823ab2c7c0b780b8868de41b810dacfc08cf600