AudioPolicy: Fix UAF of a temporary DeviceVector

AudioOutputDescriptor::devices() returns a DeviceVector
by value. That means, the returned vector must be held
as long as we need its elements.

Added a FIXME to consider other devices because the
current code looks suspicious.

Bug: 128327759
Test: make
Change-Id: Ifd3a7c8faf89b2c4fc3400aac0060d0e9e673e73