AudioFlinger: device effect not added to HAL

Regression introduced by checking on assigned io rather
than boolean addedToHAL. Device effects may not be assigned to
an io, hence they are never added to HAL.
This CL fixes by adding relying on an virtual API rather then in io.
Implementations will be specific to device / session effects.

Bug; 329395147
Test: manual
Flag: EXEMPT bugfix
Change-Id: I91a267046b494d3ce7bbf4bdeebcdf75094f5758
Signed-off-by: François Gaffie <francois.gaffie@renault.com>