Part of: #8149

Description

When we format the delta percentage to display on an Analytics card (e.g. +36% or -16%) we also need to keep track of the direction of change (positive, negative, or zero). This PR updates the stats data formatter so it returns both the formatted string and direction of change for a stats delta percentage.

It also includes a couple small fixes to the calculations.

Changes

• Adds a DeltaPercentage struct to represent the formatted string and direction of change, and returns that (instead of just a String) from the corresponding formatter methods.
• Fixes the delta calculations:
  • If the previous and current values are both 0, the delta percentage is +0% (previously it was incorrectly returning +100%).
  • If the previous value was 0 and the current value is negative, the delta percentage is -100% (previously it was returning +100%).
• Adds/updates unit tests.

Testing

These delta percentages aren't yet used in the app; confirm the unit tests are checking the correct values and tests pass.

Submitter Checklist

Update release notes:

• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.