Allow pixelflinger to work when NX (No Execute) is enabled.

Instead of allocating memory from the (non executable) heap,
allocate memory using mspace and ensure that we use mprotect
to mark it as PROT_EXEC.  This allows pixelflinger to
continue to work even when NX protections are enabled.

Testing: Using the ApiDemos market app, verify that
Apidemos -> Graphics -> OpenGL ES -> GLSurfaceView
works when "adb shell setprop debug.egl.hw 0" is set.

Change-Id: Ib569cd2543c6fa25688ee76325a712bc2347450b