Move executor details to ReadableArchive.

WriteableArchive is going to use a different executor. Files have to
be written sequentially, so in only one thread in parallel. On the other
hand, ReadableArchive supports parallel reading.

Test: Refactoring only. Unit tests pass.
Bug: 20822019
Change-Id: Ie030c2e9478be576ae87f8f30b8d1e9388507cb2