Allow access to preparers that produced a TestResult

The Preparer() method allows for another test to be run using the same
preparers as were used to create the TestResult. That is useful when
testing functionality like the sdk snapshot mechanism that generates an
Android.bp and a set of files that are intended to be reconsumed by
Soong. This method allows the test to process that output to ensure
that it is valid. A follow up change will migrate the CheckSnapshot()
method to use this as part of the migration of the sdk package to the
test fixture model.

Previously, the dedupAndFlattenPreparers() would not construct a list
containing all the preparers passed to it as it was trying to avoid
constructing such a list unless necessary. However, as every usage of
this method now needs to construct such a list that functionality has
been added to it.

Bug: 181070625
Test: m nothing
Change-Id: Iab34e2c861bdfd05c44e982224f70c5abc19e468