BufferQueues: Always respect setMaxDequeuedBufferCount

For some reason, this wouldn't be respected if no buffers had been
queued. This could lead to surprising behavior of a (with unlimited
buffers, essentially unbounded) buffers being dequeued for new BQs.

This is flag gated to capture potential issues, since I'm worried about
chesterton's fence.

Bug: 399328309
Flag: com.android.graphics.libgui.flags.bq_always_use_max_dequeued_buffer_count
Test: new tests

Change-Id: I756eb4f621f6eec1a5bb43a490305018ec69c477