Music service cleanup

[jonvmey]

@Avamander here are the changes I mentioned in #359 plus one or two other small improvements.

1. Eliminated the need for reinterpret_cast<ble_uuid_t*>ing by passing pointers to the first member of the UUID structs, which is a ble_uuid_t, instead of the struct itself.
2. Replaced the macro definitions for the UUID values with constexpr functions that return UUID objects directly and used those to initialize them. Also moved these variables to static values in the source file since that's the only place they're referenced.
3. Moved MusicCallback into the anonymous namespace since it's also only referenced inside of MusicService.cpp
4. Moved initialization of member variables with fixed constants to class declaration in the header and added explicit initializers for some uninitialized members.

Another thing I was thinking of doing was adding a simple operator== for the UUID objects, which would just call ble_uuid_cmp. That would help make the if-chain in MusicService::OnCommand a lot cleaner to read.

[Avamander]

I plan on testing this, but not sure if I'll manage it within this week.

[JF002]

Nice 👍 I like the way you replaced the macros by constexpr functions!