[Hotword] Delay checking for audio devices until needed.

[Hotword] Delay checking for audio devices until needed.

Checking for audio devices right away is causing performance regressions on Mac. This CL delays
checking until needed, but does not include the availability of an audio capture device in the
error reporting until after it's known the check has been completed. Otherwise, the settings
page immediately displays an error upon opening.

In general, this should not be a problem as typically the audio device check will have completed
by the time the user goes to the settings page. There is a rare use case, where the user
installs Chrome and immediately goes to settings and in that case, we won't be able to
accurately report if there is an audio device preventing the user from starting hotwording.
However, by the time they reload, the error should be present which should be sufficient.

BUG=385514,413104

Review URL: https://codereview.chromium.org/654613004

Cr-Commit-Position: refs/heads/master@{#300965}