refactor all usages of the Gson object to use a singleton instance - sparing costly initialization, which has resulted in major performance implications for our clients.

this is a known optimization as Gson instances are stateless and thread-safe ("Gson instances are Thread-safe so you can reuse them freely across multiple threads" https://www.javadoc.io/doc/com.google.code.gson/gson/2.9.0/com.google.gson/com/google/gson/Gson.html)

https://stackoverflow.com/questions/31432758/should-i-reuse-one-instance-of-gson-or-create-new-ones-on-demand
https://stackoverflow.com/questions/10380835/is-it-ok-to-use-gson-instance-as-a-static-field-in-a-model-bean-reuse

tests and lint pass.

in addition consolidated fromJson declarations to a single top level one in Utils to avoid code redundancy.