[Telemetry] Add telemetry for the Home screen button

[gabrielluong]

Description & Product Manager / Data Scientist User Story

As a Product Manager, I want to be able to retreive data on how many users Return to Home by tapping the new homescreen button.

User taps on the new homescreen button

What questions will you answer with this data?

Acceptance Criteria

• ENG files a DS JIRA request outlining their methodology.
• DS sign off on instrumentation methodology addressing product questions.
• Event pings can be queried via re:dash
• Event pings can be queried via amplitude
• We are sending telemetry events for the actions listed in the requirements
• We have documented the telemetry
• We have asked a data steward to review the telemetry

┆Issue is synchronized with this Jira Task

[amedyne]

Corresponding UI change is here: #19876

[lobontiumira]

Verified as fixed on the 6/15 Nightly build with Google Pixel (Android 10).
The "home button tapped" event was generated:

"category": "events",
          "name": "browser_toolbar_home_tapped",
          "timestamp": 5164

Ping: 25a9692f-28cd-44e0-9764-f3309053a0cb
Glean dashboard