x/ref: Separate impl.go and jni.go ptr maps
x/ref: Separate impl.go and jni.go ptr maps

The reason they need to be separate is that for inviteScan and
peerScan, we are using the same handle value.

ptr maps do not accept duplicate keys, and they no longer generate
unique keys for us. Thus, it is better if we use separate maps in
the jni-side and impl-side.

Change-Id: I4d892471583d5be90637c330d619fed1bb0ba59f

vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref