stagefright_foundation: Fix parsing seek pre roll in Opus

Seek pre-roll parsing was skipped in OpusHeader because of
wrong loop check.

Also, C2 opus decoder plugin was reading from codecDelayBuf instead
of seekPreRollBuf

Test: With a local AMediaCodec api based application to decode
 opus encoder's output

Bug: 123426341
Change-Id: I0924f67c4a070a582b578d2ecdcd2fecd10134ad