C2VdaBqBlockPool: set a constant maximum dequeued buffer count

Originally, we use IGBP's maximum dequeued buffer count to restrict
the total allocated buffer count. However, the maximum dequeued buffer
count only affects the free BufferQueue slot count, not related to
allocated buffer count. Also, we already track the allocated buffer
count by TrackedGraphicBuffers class.

This CL sets a big enough maximum dequeued buffer count when
configuring the producer. It simplifies the logic in general.

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

Change-Id: Ibd8859395732c85521b2e609b46af4b203d969c0