Add custom targeting for at risk users background task notification (PiP notification experiment)

[ncalexan]

This tracks adding custom targeting for background task messaging. The definition is for "at risk users" and is intended for use in the Picutre-in-picture (PiP) notification experiment.

• Windows 10+
• running background task
• >= 20 days of use in the last month (28 days)
• last active between [7-28) days ago

┆Issue is synchronized with this Jira Task