C2VdaBqBlockPool: Polish queryGenerationAndUsage method

This CL polishes queryGenerationAndUsage method, including:

- change the return type to status_t to reduce type mapping
- rename to queryGenerationAndUsageLocked to indicate the method
  should be called during locked
- remove unnecessary fence waiting

Bug: 160110407
Test: android.media.cts.AdaptivePlaybackTest

Change-Id: Ie85acde8bdfdc5b6e2620e590aed9dd606e1b507