leaudio: Improve handling microphone during call

If one device is connected, let's use this stream as left and right
channel

Bug: 245326683
Test: atest BluetoothInstrumentationTests
Test: phonecall
Tag: #feature
Merged-In: I92e43669d5d0b7f3bb87142d3e2842eee9f27b4c
Change-Id: I92e43669d5d0b7f3bb87142d3e2842eee9f27b4c
(cherry picked from commit dd34dd40)