Use menu shortcuts in ApiDemos

Add alphabetic shortcuts to the menu in ActionBarUsage. This mirrors
what is already present in a similar support lib demo.

Bug: 31045453
Test: run com.example.android.apis.app.ActionBarUsage on a device
  with keyboard shortcuts enabled (such as Chromebook). Action menu
  items should display keyboard shortcuts.

Change-Id: I9e6b04df8e2a6cbc728cb60b3706d054521da7cb