Create a filegroup module for tests/unit/src so it can be used across am: 177c11e0cf

Original change: https://android-review.googlesource.com/c/platform/packages/apps/StorageManager/+/3342944

Change-Id: I232a38d0faf44337c2c53a8f50714ce54b3a0179
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>