Fix ACTS ImsMedia crash in stress test

Modify to use single thread to process the audio read and codec
input/output to prevent the crash by asynchorounous instance access from
the multiple threads.

Bug: 261131189
Test: Voice call test with device, atest ImsMediaNativeTests
Change-Id: Iddc798e2e6ff43442d7010ada77ea59cae6eca67
2 files changed