audiopolicy: engineconfigurable: fix double free issue

Bug: 205884982
Test: build & boot

This CL fixes a regression introduced by (or revealed by)
"Move engine loading out from the AudioPolicyManager".

Change-Id: I708d621c200963f6f1292b943f5555293e6170e6
Signed-off-by: François Gaffie <francois.gaffie@renault.com>