Display shortcut modifiers in menu

Prepend relevant meta key names (such as Ctrl+) to the shortcut
displayed in a menu.

Do not prepend "Menu+" if the device does not have a hardware Menu key.

Bug: 31045453
Test: run ApiDemos (MenuInflateFromXml), select "Shortcuts", observe
  Ctrl+ prepended to shortcuts.

Change-Id: I1a38bd1baf069dd1adb24a26f89c6db6390b8b8d