Bug 796600 - make the keyboard layout menu scrollable
 1. Add LOCKED_AREA so the menu will not be easily dismissed
 2. Will do continuous scrolling if you stick to the edge of the menu
Merge pull request #10572 from RudyLu/keyboard/Bug796600-scrollable_keyboard_layout_menu_commit

Bug 796600 - make the keyboard layout menu scrollable. r=djf