Create a filegroup module for tests/unit/src so it can be used across am: 177c11e0cf am: 0043fdd365

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

Change-Id: I48c7dc75f753c9254d5b250503a5005d686eac0c
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>