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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