This change makes sure tunnel mode transforms are properly activated
upon construction, and corrects bugs with how policy selectors were being
generated for tunnel mode policies. Specifically, the source/destination could
not be empty strings, even for cases where an empty selector was desired.
Bug: 72457770
Test: GTS tests run
Change-Id: I9a9f64c34b07883a02a5c996614f958486d214fc