AudioService: fix communication device APIs

make sure getAvailableCommunicationDevices() and
getCommunicationDevice() always return null or a valid communication
device,

Bug: 265613261
Bug: 266067856
Test: atest AudioCommunicationDeviceTest

Change-Id: I1fc0bc8a372d49b0fb6c8b25fd51fbcf321bab6f
Merged-In: I1fc0bc8a372d49b0fb6c8b25fd51fbcf321bab6f
2 files changed