Copy to std::string when parsing LE audio config

* LE audio config files are load only once when BT is enabled
* Hence, processing it using std::string with copy-value will not
  hurt performance too much while making code simpler and memory
  check safer

Test: atest CtsBluetoothTests
Test: atest --host bluetooth_le_audio_test
Bug: 224933710
Change-Id: I5281da7abce803c33db9448d697b3b9e6fc4d1eb