Fix "always" option in ResolverActivity

mUnfilteredResolveList was incorrectly being set as null whenever
performSecondaryResolveListFilterin didn't end up filtering anything
(when its value should have been maintained in this case).

This incorrect mUnfilteredResolveList null value causes most of
ResolverActivity's onTargetSelected() logic to be skipped, thus it
doesn't notify the package manager of the 'always' selection.

This should match the behavior before ag/17509713 and fix the bug.

Test: Manual testing (install two email apps, click an email addr link,
choose a resolver target and press 'always', then do it again and
resolver shouldn't appear).
Test: atest
CtsDynamicMimeHostTestCases:android.dynamicmime.cts.PreferredActivitiesTestCases#testModifyGroupWithoutActualGroupChanges
Test: atest ResolverActivityTest

Bug: 230149644
Change-Id: Ia23e95359d320a3a63e39f9c50a159879e94bf3b