Write data returns HealthDataPoint? object instead of boolean

[agilst]

IMPORTANT: It is mandatory to merge both #1193 and #1194 before merging this PR as it's depend on getHealthDataByUUID method in #1193 and #1194 PRs.

writeWorkoutData is now returns HealthDataPoint instead of boolean. Failed writing health data will return null instead.

[iarata]

@agilst I'm thinking, the way this is implemented now might result in faster API rate limits on the device, since now you are doing WRITE & READ instead of just WRITE. Wouldn't it make more sense to generate the object before writing and then return it when writing is successful and null otherwise?

[agilst]

@iarata you're right. At this stage, I don't think generate the object before writing is a good idea as we already have getHealthDataByUUID. Probably only returns the health uuid or null is all we need then can freely decide to utilise getHealthDataByUUID if need the health object. I think that is the best way to keep the health object structure persistent.

I'll proceed with the changes if you agree. or let me know if you have another suggestion.