Only send Java Enabled "je" flag if we have a value.
Merge MeasurementActivityExtensions with MeasurementAnalyticsClient.
Remove unnecessary IMeasurementActivity and ICustom* interfaces.
Move NonInteraction from event to MeasurementActivity as can apply to all types.
Update package build numbers to 1.2.0.0 for upcoming release.
Preperation for 1.2.0 NuGet release.
Update README for 1.2 release.