A new BuildConfigFields with all of the fields from this object, combined with
all of the fields from the specified fields. If both objects have fields with the
same name, the entry from the fields parameter wins.
Creates the Java code for a BuildConfig.java file in the specified javaPackage.
source - The build target of the rule that is responsible for generating this
javaPackage - The Java package for the generated file.
useConstantExpressions - Whether the value of each field in the generated Java code should
be the literal value from the BuildConfigFields.Field (i.e., a constant expression) or a
non-constant-expression that is guaranteed to evaluate to the literal value.