Hold a reference to the allocated buffers in GraphicBufferAlloc

Store the allocated GraphicBuffers in a vector list. This list is
cleared only when SurfaceTextures requests that the buffers be freed.
Premature deletion of GraphicBuffers and therefore deallocation of
gralloc handles are prevented.

This change partially reverts:
commit 5f05f99a
Author: Mathias Agopian <mathias@google.com>
Date:   Fri Apr 8 19:10:43 2011 -0700

    Fix a GraphicBuffer leak in SurfaceTexture

    This leak was intentional, it was there to deal with the fact that
    some gralloc implementations don't track buffer handles with
    file-descriptors so buffers needed to stay alive until there were
    registered, which is not guaranteed by binder transactions.

Change-Id: Ia732806bf8d8d15b66944569e8aa2120641d9812
CRs-fixed: 327655