ContextHubService: Don't change app handle on load

Due to the way this logic was set up, every time we would load
a new nanoapp, add_app_instance() would change the nanoapp's
handle/"id" in manner that was invisible to closeTxnLoad(),
so our JNI cache would use a different handle than what we
passed to the Java callback.

We remove this unnecessary changing of the handle/"id".

Bug:31325144
Change-Id: I8678729f38fb804bc6a128ce9722fd35551fa1c1