gralloc: Use getpagesize() in place of PAGE_SIZE.

- Use getpagesize() function from unistd.h to get page size
  in place of PAGE_SIZE macro.
- Make isDisableUBWCForEncoder declaration pure virtual.

Change-Id: I89405e999e4683874f4daa32188181e688a19812
CRs-Fixed: 1069351
diff --git a/libgralloc/alloc_controller.h b/libgralloc/alloc_controller.h
index 8216b0c..45977e2 100644
--- a/libgralloc/alloc_controller.h
+++ b/libgralloc/alloc_controller.h
@@ -56,7 +56,7 @@
 
     virtual IMemAlloc* getAllocator(int flags) = 0;
 
-    virtual bool isDisableUBWCForEncoder();
+    virtual bool isDisableUBWCForEncoder() = 0;
 
     virtual ~IAllocController() {};