Filter non-exported components from activity start

This change filters non-exported components from resolveForStart queries
to avoid permission checks done as part of activity starts. Since these
components are not intended to be used by other apps, we'll hide them
at query time instead of adding complex enforcement at permission check
time.

Bug: 197945962
Test: atest AppEnumerationTests
Change-Id: Ib066a04ac30efa542e4efa62201d0937435e923c