Reapply "Add unit tests for LoaderTask"

This reverts commit e4735d77f11d8659b82a4ef4e478d1343162fb47.

Bug: 301105199
Test: This already passed presubmit before.
Change-Id: I97ab28186ba1d4da4abe82686fe1a11d5e51c4b3
4 files changed