[Reader IA] Implement analytics events for drop-down menu

[RenanLukas]

When Reader drop-down menu is opened
Key: "reader_dropdown_menu_opened"
No parameters.

When Reader drop-down menu item is tapped
Key: "reader_dropdown_menu_item_tapped"

Parameters:
// list should be sent when any custom list item is tapped
{ 
    "id": "discover|following|saved|liked|a8c|list"
}