Allow createGlobalSoongConfig() to be used from tests

Previously, the createGlobalSoongConfig() function was explicitly
prevented from being used in tests because it would fail. However, it
turns out that is no longer the case and it does now work.

That allows the following changes to be made:
* Tests no longer need to use GlobalSoongConfigForTests() to
  prepopulate the cache.
* GlobalSoongConfigForTests() is only needed in the dexpreopt
  package.

Bug: 177892522
Test: m nothing
Change-Id: Ifcbb1a44254c5d2d10c1d02ab23227488d1d1ed1