Set stability for unset interfaces.

This specifically has handwritten interfaces in mind. This CL ensures
that if there is a handwritten interface, it'll be marked with 'local'
stability before it is sent out. This means that a handwritten interface
won't accidentally be interpretted as a stable interface.

Bug: 136027762
Test: boot
Change-Id: I97e3e7c72f99e12dbf00996ff0c68fb683f3c494