Adds tracking when expanding items on the Stats: Post Stats view

[emilylaguna]

Project: #17503
Depends on WPiOS Shared PR: wordpress-mobile/WordPress-iOS-Shared#303

To test:

1. Launch the app
2. Tap on the My Site tab
3. Tap on the 'Stats' item
4. Tap on an item under 'Posts and Pages' to open the 'Post Stats' view
5. Under Months and Years tap the down arrow to expand a section
6. Verify you see: 🔵 Tracked: stats_posts_and_pages_months_years_item_tapped <blog_id: BLOG_ID, site_type: SITE_TYPE>
7. Under Recent Weeks tap the down arrow to expand a section
8. Verify you see: 🔵 Tracked: stats_posts_and_pages_recent_weeks_item_tapped <blog_id: BLOG_ID, site_type: SITE_TYPE>

Regression Notes

1. Potential unintended areas of impact
   None

2. What I did to test those areas of impact (or what existing automated tests I relied on)
   None

3. What automated tests I added (or what prevented me from doing so)
   None

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

You can trigger an installable build for these changes by visiting CircleCI here.

[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[momo-ozawa]

Works as described!
Approved the corresponding WPiOS Shared PR 😄