com.android.server: 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: com.android.server finds headers
Bug: 63762847
Merged-In: Icb3018f520004f65e8dd565d6e1d563551548eba
Change-Id: Ib36fec53c30760af5dac4097b3eceda6c4c6c7f0