Mark AggregationResult#get with ObsoleteApi annotation

[jainsahab]

Context: With the introduction of SUM And AVG aggregation, we have exposed two new methods AggregationResult#getLong(String) and AggregationResult#getDouble(String) as the existing method Long AggregationResult#get was not enough for the cases where we get an aggregation result of Double type (like in SUM and AVG aggregation). Now users can call the respective methods (getXXX) based on the type of aggregation results they are expecting.

Action Item: Once ObsoleteApi annotation is released and ready to use, mark AggregationResult#get with it.