BufferItemConsumer: Add/expose methods

Removes unneccessary flag surrounding releaseBuffer, which is flag gated
everywhere else.

Adds attachBuffer, which wraps IGBP's version of the same method,
allowing users to attach a buffer to the consumer.

Bug: 393639203
Flag: EXEMPT covered elsewhere
Test: new tests

Change-Id: I13283720f0f91380301d1aca180c4681c16997d7