Currently we memset MicroResourceVariables buffers to 0 if it is null, and in reset we memset to 0 again.
This change makes it so that we memset to the zero_point instead.
To get the information of the zero_point value while reset, we need to store it in the MicroResourceVariable class.
BUG=[269648474](https://b.corp.google.com/issues/269648474)
* Add method to reset resource variables.
Call this method from ResetVariableTensors.
* handle case where resource_variables_ is nullptr in micro_graph