native: Cleanup jni usage code

Make the C++ signature consistent with the JNI descriptor by using the
same type as the descriptor whenever possible.

e.g. "()Ljava/lang/String;" should actually be "jstring fn(JNIEnv*,jobject)"
instead of "jobject fn(JNIEnv*,jobject)".

Bug: 35325126
Change-Id: I72318525fc3b18b013b8d6fa604d8dd6b5dd4400
Test: make -j32 test-art-host  # and also manually that it boots