libgui/SurfaceFlinger: Add getConsumerName

Adds a getConsumerName method to IGraphicBufferProducer and Surface.
Currently, the name is cached inside of IGBP and is update on connect
and dequeueBuffer, which should be good enough for most uses.

Bug: 6667401
Change-Id: I22c7881d778e495cf8276de7bbcd769e52429915
(cherry picked from commit c6f30bde)