Force overflow menu icon to be shown.

...even on devices that have a physical menu button.
Note: does not work on 2.3 devices.

Also removed key handler for the Menu button to bring up the Nav menu, so
that users with 2.3 may access overflow items (or indeed users who are
used to pressing the Menu button to access overflow items).

Change-Id: I2bf46f4ff3e8e952e193addceec4998a95aeefd9