Decouple sdkextension tests from their baseclass am: cd8df6cd38 am: cdf56cb111 am: 5c3ccde6fd

Original change: https://android-review.googlesource.com/c/platform/packages/modules/SdkExtensions/+/1505636

Change-Id: Icd7b965796b220f329094d149f0936e0c14b97fa