Add JNI helper function jniCreateString().

In C++11, char16_t and uint16_t (which is what jchar is defined as) are
two distinct types, even though they are bitwise identical, so to not
require C++11 code using char16_t to always reinterpret_cast<uint16_t>
the argument passed to JNIEnv::NewString(), this new helper function is
overloaded to do the required cast when used in C++11.

Bug: 37554848
Test: make
Change-Id: I69632b0a7b822537cab77c43dc387d94917933b9