NLS: Add a public signal value for an undefined filter value.

We have three possible defined values for getInterruptionFilter().
i.e. All/Priority/None.

However, this value is only returned to listeners once connected,
otherwise we return 0, an undefined value.

This change gives a name to this undefined value to make it clear
that callers should not infer any meaning from it.

INTERRUPTION_FILTER_UNKNOWN = 0;

Bug: 19288429
Change-Id: I8ae94d1723289ca5714800906f9bf4e7e8111813