Implement issue #1780928: Need support hiding nav keys.

This implements support for devices whose hardware can hide
their navigation keys.  It works much like the existing keyboardHidden
configuration, and for compatibility uses the same configuration
change bit.

Also add FLAG_TURN_ON_SCREEN for windows, which has the system
cause the screen to be turned on when the window is displayed.
Great fun when used with FLAG_SHOW_WHEN_LOCKED!

Change-Id: I0b867f19af85cfd8786a14cea194b34f7bdd9b7a