[Android] Use Java long for JNI pointer

- replace all stashed pointers in java from
"int mNative" to "long mNative".

- change all casts from "reinterpret_cast<jint>"
(and other variations) to "reinterpret_cast<intptr_t>"
that would then be expanded to jlong.

Upstream switched to long by default so we don't need
to special case our JNI generators anymore.