Refactor storage of runtime exclusive storage flag in QA

This is a follow-up for “qa_config: Remove exclusive storage flag from
config”. Instead of storing the flag in a module-level variable it is
now stored within the new QA configuration class and unit tests are
provided. Wrappers in “qa_config” maintain the existing interface.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>