Fix AudioPolicyManager dynamic policy initialization

AudioPolicyManager::getInputForAttr() is initializing the AudioMix
 for the new AudioInputDescriptor.
The audio policy manager refactoring introduced a regression where
 the association of an existing AudioMix (registered when the
 associated AudioPolicy was registered) with the input descriptor
 copied a value of local scope, instead of referencing the
 AudioMix in the list of mixes.

Bug 20081375

Change-Id: Iddd57fed8525880afd1d4a5493f97130aabe4816