BufferQueue: Increase MIN_UNDEQUEUED from 2 to 3.

If 2 queueBuffers happen fairly closely, the former buffer is not picked by
display. But SurfaceTexture won't return this to clients, since its not the
oldest. It will return the locked buffer causing deadlock.

Increasing the min_dequeued buffer count to 3 (from 2) will increase the
distance between video and display by 1 more.

CRs-fixed: 382393

Change-Id: I275c01473c7057714da35693dfda7a358e9d9285