fix: serialisation happening on a separate thread executor service

Fixes (https://github.com/rudderlabs/rudder-sdk-android/issues/395)

Type of change

[x] Bug fix (non-breaking change which fixes an issue)
[ ] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
[ ] This change requires a documentation update
Checklist:
[ ] Version upgraded (project, README, gradle, podspec etc)
[x] My code follows the style guidelines of this project
[x] I have performed a self-review of my own code
[x] I have commented my code, particularly in hard-to-understand areas
[x] I have added unit tests for the code
[ ] I have made corresponding changes to the documentation