Add '-unstable' to solve ODR violation

The interface which is imported by an unstable interface is 'unstable'
as well. Until now, the ODR violation checker in aidl has omitted an
interface imported, but it will be checked, accordingly, fix the current
problem.

Bug: 146436251
Test: m nothing
Change-Id: I997a52d883df720711e6c5defced465952951750