Rename unfold background executor to add single thread assumption

In code, we assume that the background executor provided uses a single thread. This makes it more explicit by changing the annotation.

+ small dagger usage enhancement: we were manually creating some object instead of using dagger for them.

Bug: 261320823
Test: Failures should happen at compile time, if any
Change-Id: I3b1cff2ad2072f784085bf8d8c2067753d1e1fae