hiddenapi: Turn CHECK into a warning

We create a dummy hiddenapi-flags.csv file for ART buildbots but
Recent change to `hiddenapi` started enforcing that every dex member
has a corresponding CSV entry. Turn the CHECK into a LOG(WARNING) for
the time being to unblock the bots.

Test: rely on the ART Buildbot
Change-Id: I02565a0f11039dc28d395872d99adc7067d7e426