jni/a2dp: Cleanup A2DP initialization

- separate A2dpNativeCallback from A2dpNativeInterface
- query BluetoothCodecConfig / BluetoothCodecType methods
  in JNI load function rather than lazily

Bug: 331817295
Test: atest BluetoothInstrumentationTests
Test: m com.android.btservices
Flag: EXEMPT, minor refactor
Change-Id: I80984a36d864884f7a347076e7a78927555f9607