Mobile FTUE: Add Analytics tracking for Notifications (iOS)

How can we measure how many users have Notifications turned on?

Can we measure _when_ notifications are turned on, as well as _if_ notifications are turned on?

Notifications on iOS are 2-fold; Element Settings & iOS Settings.
Users may have Element Notifications on in the Element app, but off in iOS settings so our notifications may not be delivered. It would be **great** to be able to measure whether Notifications are on and whether Notifications are deliverable/delivered.

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The issue concerns analytics for Element notification settings, iOS notification settings, and notification deliverability. No files, tests, or entry points are named; first identify the existing notification and analytics code, then clarify which states and events should be measured. Done requires an agreed tracking design for enabled state, change timing, deliverability, and delivery.

Written by the indexing model from the issue text.