VCP: Split nativeInterface from nativeCallback

One is going down to the native, the other from. This allow to have a
couple final method in the nativeCallback and remove the need for null
pointer check (by design)
Similar as HAP implementation
https://r.android.com/2953314 -> direct use of native interface
https://r.android.com/3199943 -> callback split

Bug: 311772251
Bug: 361263965
Test: atest BluetoothInstrumentationTests
Test: atest VolumeControlNativeCallbackTest
Flag: com.android.bluetooth.flags.leaudio_add_aics_support
Change-Id: Ia167c8db9f5de5f9dd6508abde1bf9b8e4b0cbb8