C2VdaBqBlockPool: avoid unnecessary allowAllocation call

This CL tracks the current status of allowAllocation(). If the target
status is the same as the expected status, then we skip the call.
Also, this CL disables allocation when we dequeue more buffers then
requested.

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

Change-Id: I860db3be92b06f18c7f5a0e7b6a391ccd26bb6f6