audiopolicy: fix preferred device selection

Make sure preferred device selection is taken into account
in getOutputForAttr() even if called twice on the same session:
Force changed flag when adding a SessionRoute even if
the same device is specified.

Also renamed SessionRoute isActive() to isActiveOrChanged() and
hasRouteChanged() to getAndClearRouteChanged() for clarity.

Bug: 69680975
Test: manual
Change-Id: I3f3a659568c4dfc825a788753b0302136fd89871