[Refactor] Rename method add to addTask

Gives it more readability and intuition.
[Refactor] Extract methods for adding tasks to Google

High-level methods related to adding tasks to Google falls is extracted
and moved to ConcurrentAdd
[Refactor] Extract methods for modifying tasks to Google

This commit extracts the relevant methods from GoogleController and
ConcurrentHandler to put them into the ConcurrentModify class where
modification takes place.

It also add some constant Strings to remove magic strings.