Keyboard shortcuts: icons for shortcut items

Introduces the ability to display a shortcut item as
an image (if it has a resource attached). Also added
vector drawables for: up, down, left, right, enter, meta
and backspace.
Note that accessibilty wise the drawables dynamically scale
to the height of the text items (which have the text size
set in sp).

Bug: 27455410
Change-Id: I49739313d83cf661bea9378108a1fa3d6c51bd2e