pixelflinger: make self-contained

pixelflinger should not be used for new projects and should be moved out
of system/core at some point.  As the first step, move all its headers
under system/core/libpixelflinger/include and update its Android.mk
files so they're not referring to the absolute system/core path anymore.

Change-Id: I7acc3ef5b2e21d8ba6fe939ea8d3ec4c41959591
Signed-off-by: Greg Hackmann <ghackmann@google.com>