Fenix: [UX] Easy access to top share app

[vesta0]

User Story

• As a user, I want a quick easy way of sharing to the app I most frequently use for sharing, so I can save time.

In both Chrome and Fennec, the top share app shows up as an icon in the browser toolbar menu.

Acceptance Criteria

• I can quickly share a page to the app I most frequently use for sharing.
• [Telemetry Ticket] Track how often users choose to tap on the top share app.

[BranescuMihai]

@vesta0 Isn't this the same as #4231?

[vesta0]

Not exactly but related! This is about quick access to the top share app only.

Example from Fennec (Pocket being my top share app):

[AmyYLee]

Assigning to @topotropic