audio policy engine: fix ringing over SCO device selection

The request to ring over SCO only (not speaker + SCO) should only
be honored if SCO is the preferred device for phone strategy, indicating
that SCO audio is actually active.

Bug: 374111953
Test: manual
Flag: EXEMPT bug fix
Change-Id: I2ff9818f2f225d519ce6c10406a59c654b679fca