Bug 1011669 - [B2G][Browser]Double-tapping on plus sign (New Tab) presents more New Tabs than what shows when returning to New Tab screen

* Only respond to new tab clicks if not in transition and the current screen is TABS_Screen to prevent multiple empty tabs from created
Merge pull request #24318 from begeeben/1011669_double_tapping_on_plus_sign_master

Bug 1011669 - [B2G][Browser]Double-tapping on plus sign (New Tab) presen...r=benfrancis