aapt2_jni: 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 aapt2_jni find headers
Bug: 63762847
Change-Id: Id51354d533bfef66ecd71f5d89599b8adc688c1f