This introduces test coverage, using the Glean SDK
testing API, for the metrics that are set at startup
by Fenix in the GleanMetricsService.
This additional adds a basic test for the translation
of the `app_opened` event.
This fetches the Google Advertising ID, salts it and
then applies hashing before sending a ping with it,
at startup. Hashing and salting are used in order
to prevent ourselves to correlate advertising IDs
from the same user running different products we
own off a single device. We will never send the
client_id and the Google Advertising ID in the same
ping.