Introducing the split action bar.

Apply (or extend) the theme Theme.Holo.SplitActionBarWhenNarrow or
Theme.Holo.Light.SplitActionBarWhenNarrow to enable splitting the
action bar across both the top and bottom of the screen. This places
the action menu along the bottom, leaving more room at the top for
titles, navigation, and custom views and more room at the bottom for
menu items.

TODO: Refine layout of the action menu when placed at the bottom of
the screen. Make action modes split as well.

Change-Id: I92c91f99c533f26aecf6b828ed041386c4f16922