hdp: do not return a null value when hdp interface unavailable

We're not supposed to return a NULL from a int return function.

And indeed the calling Java code does not have a NPE check either.

If an HDP interface is not avaialable, send -1 which the Java
code correctly assumes as a failure to register the health app.

Change-Id: I5ac7dc6f9acc9fefae8d1d2233453448c3ba79bc
Signed-off-by: Arnav Gupta <championswimmer@gmail.com>