leaudio: Extract target latency to the json file

With this patch we can set target latency per direction in json file.
Default is balanced

Bug: 150670922
Bug: 213255212
sponsor: @jpawlowski
Test: atest --host bluetooth_le_audio_test
Change-Id: I99529a27731b2e5bc3a9b2493fa2f1a46514a776