Add aconfig flag for wearable hotword

Merging the flags first allows us to promote them to staging before the
feature code is merged, which in turn allows the tests gated by these
flags to be run as part of the feature CL's presubmit when the feature
and tests are submitted together in the same topic.

Test: Will rely on presubmit
Bug: 310055381
Change-Id: Ie1248eb3c4844922d5656d4f49e290c8f0243442