Add BufferItemConsumer, a simple BufferQueue consumer.

BufferItemConsumer allows for acquiring BufferQueue's BufferItems,
which contain all the data and metadata the BufferQueue has for a
given graphics buffer.

This consumer is useful when direct access to the native buffer_handles
is needed by the client.

Also includes a minor cleanup of CpuConsumer's use of 'virtual'.

Bug: 6243944
Change-Id: If7dc4192b15ac499555f1eda42a85140f2434795