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: Idead273ab2c0450409d770f5402c4dba916192a9
Signed-off-by: Greg Hackmann <ghackmann@google.com>