The exception was occurring because of the missing guava.jar file on the build machine -
/apps/SampleApp/android/native/libs/guava.jar
which got generated after an upgrade from Worklight 6.0 to 6.1.
We have followed the worklight doc - Integrating with source control systems which doesn't mention anything about "libs" folder and hence didn't checked-in the file.
Now build machine takes whatever checked-in and builds the APK file. The generated APK file doesn't contain "guava.jar" file, which contains package "com.google.common.collect.Maps". And hence we were seeing the issue.
Note: We were seeing the issue after adding changes to use JSONStore. So it looks like JSONStore use this package.