stagefright: allow connecting to surfaces that attach buffers

Now that consumers can attach buffers to BufferQueues, we cannot
assert if an unknown buffer is dequeud, or if a buffer is dequeued
from a BufferQueue multiple times.

Also, when attaching to such surfaces, allocation must be enabled
as attach will lock for a free buffer slot.

Bug: 20885565
Change-Id: Ied466c5a848facf3149ad8bf0d18a0095da21e40