APS: initialize portId on |getOutputForAttr|

|portId| is uninitialized when checked in
|AudioPolicyManager::getOutputForAttr|. This CL initializes the variable
for safety.

Bug: 367117623
Test: m
Flag: EXEMPT trivial bug fix
Change-Id: I4d9e7a02c56374c94424bffa0afba712e56b3863