Rename FixturePreparers to GroupFixturePreparers

FixturePreparers is a bad name for a method that creates a collection
of FixturePreparers for a couple of reasons:
* Conventionally it would be used as the name for []FixturePreparer if
  it is necessary to add behavior to that.
* There are many different types of collection, particularly when order
  matters.

Bug: 181070625
Test: m nothing
Change-Id: I55394ff369375dcac2d7b72e4d803a4818762d36