Keyboard shortcuts: UI polish 1

* title color for system groups is now: "material_deep_teal_500"

* background behind shortcut keys:
** Now has 2dp rounded corners
** changed color from "material_grey_200" to "material_grey_100"

* the text items now have a minimum width equal to their height.
This means that now the text items with one character are always
the same (square) size as the icon items. Makes the UI look much
cleaner thus easier to read

* the line item now has a minimum height of 48dp and the content
is vertically centered

* minor variable renaming for increased readability

Bug: 28075364
Change-Id: Id7090b607b9c604c55513e7c393ed1084a1c8df0