Add implementation for unimplemented APIs and add description for return value

1. Add implementation for getCodecStatus(), setCodecConfigPreference() and isCodecConfigSelectable()
2. Add description for the return value of getCodecLocalCapabilities() and getCodecSelectableCapabilities()

Tag: #refactor
Bug: 219875113
Test: atest BluetoothInstrumentationTests
Change-Id: Ida32ff52990d2c062ef9f5cc965e20af030896bd