merge libsurfaceflinger_client into libgui

this is the first step in unifying surfacetexture and surface.
for this reason the header files were not moved, as most of them
will eventually go away.

NOTE: currently we keep libsurfaceflinger_client.so as an empty
library to workaround prebuilt binaries wrongly linking against
it.

Change-Id: I130f0de2428e8579033dc41394d093f4e1431a00