SharedMemory cpp: use proper nativehelper headers

libnativehelper exports headers under nativehelper. These were
available before incorrectly as global headers in order to give
access to jni.h.

Test: cpp android.os.SharedMemory finds headers
Bug: 63762847
Change-Id: Ib1680c1f0508b457bbe17ca63375a53eeb071a25