Update to use nativehelper/JNIPlatformHelp.h

Reflects refactoring of JNI helper code that depends on private
methods within libnativehelper.

Bug: 151443957
Test: Treehugger
Change-Id: Ic174c63462da4613857c30e92739b5781dcda753
Merged-In: Ic174c63462da4613857c30e92739b5781dcda753
Exempt-From-Owner-Approval: cherry pick
(cherry picked from commit aa614cc7)