Change wording of audio processing message body.

Re-wording the audio processing message body to make it more clear that
the app has not actually "placed" as call, but that it is instead simply
processing a call in the background.

Test: Manual test using audio screening to verify message.
Test: Build
Fixes: 240479337
Change-Id: Ib072678697f71bd0682b87dfb58f0c6a622d838a