externalNativeBuild for the test application

[tobrun]

Follow up from #8885. Instead of excluding the example-custom-layer from release builds we should introduce an additional externalNativeBuild for the test application. This would build the example-custom-layer target and as a result won't include the example-custom-layer.so in the library.

[stale]

This issue has been automatically detected as stale because it has not had recent activity and will be archived. Thank you for your contributions.

[stale]

This issue has been automatically detected as stale because it has not had recent activity and will be archived. Thank you for your contributions.