Expose device charging state as entity with battery_charging device_class

[timvdalen]

Describe the feature

Currently, when an iOS device has battery level <20%, but is charging, it is shown as a problem on the Overview dashboard in the Maintenance chip.
In home-assistant/frontend#52238, we're intending to hide charging devices from this chip.
However, the mobile_app platform doesn't expose charging state in the way that frontend is expecting.

For context, this is what it should look like when an iOS device is charging but at 19% battery:

frontend expects an entity with device_class battery_charging that is a binary sensor with state "on" when the device is charging.

Use cases

Charging state for iOS devices would be normalized with other types of devices, and could be used in future generated dashboards without any platform-specific changes.

Examples

No response

Anything else?

No response