ContextHubService: Make app handle size match Java

Our code was sometimes using 'uint32_t' and mostly using 'int'
to represent these custom handles.  It was also passing this
value, as a byte stream, to the Java layer.

In the Java layer, this is an 'int', and thus we change all
of our usages to be 'jint' here.  However, we still pass it
as a byte stream, since we're too late in the release cycle
to change the API.  But now this is at least a consistent
size for the code.

This code still suffers from hub handles being inconsistent
with their types, but we leave that for another bug (b/30958512).

Bug: 30806218
Change-Id: I8d9ae8b9519f399d6723cf96293d84a2f5bd9cce