get rid of purgatory and fix QueuesToWindowComposer query

the purgatory list wasn't needed anymore; in fact it had no effect as
buffer life-time management is now handled by the BufferQueue.

For QueuesToWindowComposer we keep a list of wp<> on the IBinder
for IGraphicBufferProducers we hand over to clients so we can
easily check if an IGraphicBufferProducer is ours. We clean-up the
list when our IGraphicBufferProducer are destroyed.

Bug: 8349142
Change-Id: I1aa06652ade8c72d0004a3f5e6c3d6e8a82fc2ae