libstagefright: Stop using IGBC::BufferItem

Switches all uses of IGraphicBufferConsumer::BufferItem (and
BufferQueue::BufferItem) to the BufferItem in libgui. Depends on
frameworks/native I699ed0a6837076867ca756b28d1ffb2238f7a0d9. This
is a member of I187b3a7d05196b6289596afac8fb9a9d4aebff76, but since
it was split out from the camera change in frameworks/av, they have
to have different change IDs.

Change-Id: I0ddd38df37500cfd6b21d1e768ed14e39c5cd9fc