C2VdaBqBlockPool: wait fence just before returing the buffer

Currently we wait the fence of the buffer with timeout. To reduce the
chance of timeout, we move the waiting call to the last point, just
before returning the buffer.

Bug: 178770649
Test: android.media.cts.AdaptivePlaybackTest
Change-Id: Idf2b4a40397a6632120e6d04c1b95b97933de457