ContextHub: Avoid reference leaks

We were leaking references in each call to onMessageReceipt(),
leading us to eventually run out of references and crash.  We
now delete our local references prior to leaving the function.
Also, since especially our message size can be quite large,
we put in logic to gracefully fail if the system is out of
memory.

In addition, we fix up reference leaks within
constructJContextHubInfo().  These were one-time leaks at
initialization, so not the root cause of our crash, but still
unnecessary leaks which are easily fixed.

Bug: 28655628
Change-Id: I3a3dc8e069c6c1810f3152872d8d8410e8ad0683