AudioSystem: Fix race condition in accessing ioDescriptors

The vector mIoDescriptors can be simultaneouly modified
and accessed by 2 threads. Acquire a lock while wrapping
the ioDescriptor in a sp<>

Bug: 24576810

Author: Haynes Mathew George <hgeorge@codeaurora.org>

Change-Id: I73c79ef8eca092b500a7ead3a5ebd0bcf75f9920