feat: flush events on sent to background

Description

Feature: Make a flush call each time when the app moves to the background. Also, added test cases for this feature.
Fixed the flush call instrumentation in the sample app.

Fixes # (issue)

[ ] Bug fix (non-breaking change which fixes an issue)
[x] 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)
[ ] My code follows the style guidelines of this project
[ ] I have performed a self-review of my own code
[ ] I have commented my code, particularly in hard-to-understand areas
[ ] I have added unit tests for the code
[ ] I have made corresponding changes to the documentation