C2VdaBqBlockPool: polish queryGenerationAndUsage() method

queryGenerationAndUsage() could access |mProducer| member, so we don't
need to pass the argument into the method.

Bug: 160110407
Test: android.media.cts.AdaptivePlaybackTest#testH264_*

Change-Id: I40cad58b5ff9cfbff03f172ec119d530b5248b63