dalvikvm.cc: 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: modules using dalvikvm.cc find headers
Bug: 63762847
Change-Id: I24776efb77b7d86d3b9d2ef78bc17bffe2e02f33