bluez5: handle XAPL battery level per Apple Guidelines

Corrects the handling of AT+XAPL Apple extensions to send
`+XAPL=iPhone,2` instead of `+XAPL: iPhone,2` based on Apple's
documentation.

Replaces magic numbers with enum values.

[Accessory Design Guidelines](https://developer.apple.com/accessories/Accessory-Design-Guidelines.pdf)