Give instructions during setup for protecting app from Xiaomi/Redmi power management

While working on #1743 (closed) I found that the Snooze app was killed when running overnight on the Redmi Note 7. Locking the app to the recent apps list prevented this from happening. We should recommend this during account setup, as we do for Huawei's protected apps and app launch settings.

(The user can also change the app's background setting to "No restrictions", which is recommended in various places but didn't help in the case of the Snooze app.)