[Hotword] Reverse the experimental flag so that it is on by default.

This means that v2 hotword (using the shared module instead of the CWS
extension) will be the default.

Updates tests to account for new default.

The next CL will take care of cleaning up usage of IsExperimentalHotwordingEnabled().


Review URL: https://codereview.chromium.org/774003002

Cr-Commit-Position: refs/heads/master@{#306730}
10 files changed