refactor: ADK's ObjectMapper shared instance is mutable (i.e., not a constant immutable object) and, therefore, its name should have the capitalization of a constant's name; this CL aligns with other code files where it is referred as "objectMapper"

refactor: ADK's ObjectMapper shared instance is mutable (i.e., not a constant immutable object) and, therefore, its name should have the capitalization of a constant's name; this CL aligns with other code files where it is referred as "objectMapper" #2485