leaudio: Use fixed audio session configuration while connected

With this patch, ENCODING and DECODING audio session will have fixed
configuration.
ENCODING session is configured for stereo 48kHz
DECODING session is configured for mono 16kHz

This is because of lack of the reconfiguration way of the existing
audio session in the Audio Framework.
Current solution, which was closing and reaopening audio session
introduced big delays and some other issues.
It was especially visible in Media->Phone switch use case.

Current approach is that resampling will be handled inside the Bluetooth
stack. For the moment, we are using encoder pitch value to resample from
48 to 16khz if needed.

Bug: 150670922
Test: atest --host  bluetooth_le_audio_client_test bluetooth_le_audio_test
Tag: #feature
sponsor: jpawlowski@
Change-Id: I1e183dcee2b9e4cb8f3099d2c94087ca4ea21d41