ScopedUtfChars: 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 ScopedUtfChars find headers
Bug: 63762847
Change-Id: I5e9f2b56f569b83ece0d854f7066655e1d64cc65