Fix minor bugs with tunnel mode implementation

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