v4.0.1 RC1

This is a pre-release build, containing only a bugfix for applications that change the root activities of their applications after instantiating a Mixpanel object. The changes should not affect the majority of users.

The changes in this release will prevent crashes when the library attempts to show a survey with a parent application that does not exist. Applications that have this behavior should explictly call checkForSurveys()/showSurvey() to show Mixpanel survey content.

This pre-release will not be deployed to Maven central, but the AAR file is attached to this Github announcement, and can be used along with the source code.