Add callback to openFit() function

[kmlzielinski]

nothing happens when you call method openFit() and you don't have Google fit installed so I added callback like is in other methods

[aboveyunhai]

@kmlzielinski can you change to async/await syntax instead of callback. So you can simply do

 const result = await openFit();
//or 
openFit().then((msg) =>{}).

callback argument is slowly removed due to deeply nested situation and inconsistency format between code.
Extra isEnabled() can be avoided in this case.