ContextHubService: Avoid uninitialized handle

We avoid ever using uninitialized stack for the value of a handle
which we pass up to the Java layer.

In cleaning up this code a little, we make more explicit a failure
case which already existed: The Context Hub properly loading a
nanoapp, but our Service code failing to set up the infrastructure
to track it.  We chose to tell the Java layer we failed in this
case, and more importantly, provide a consistent handle value.

Note that the INVALID_APP_ID is not known by the Java layer, but
it is consistent so future Java code could react to it.  At
the very least we will now always have consistent behavior when
this situation happens.

Bug:30795236
Change-Id: Id4eada529aa1b223867a47985ef1d5c1ba346ea3