Deprecate FixtureFactory in favor of FixturePreparer

Prior to this FixtureFactory was basically a FixturePreparer with the
additional support for buildDirSupplier. This makes that a reality by
moving the functionality of FixtureFactory into FixturePreparer and
making FixtureFactory extend that with buildDirSupplier.

This also deprecates the Extend() method in favour of simply using the
GroupFixturePreparers() method.

Once all usages of buildDir have been removed so the buildDirSupplier
is no longer required then FixtureFactory will be removed altogether.
In the meantime follow up changes will migrate those tests that do not
require a buildDirSupplier to just use FixturePreparers in preparation
for the eventual removal of FixtureFactory.

Bug: 183235980
Test: m nothing
Change-Id: Ibb4e5bfec2f76ee34dd7de6aed6bbbfe27d337b8