2D Recents: fix toggling behavior

This changes the toggling behavior so that a quick double tap on
the Recents button still does the same thing as Alt-Tab, but if
the second tap is slow, we go back to the most recent task.

Bug: 34636734
Test: Checked toggle and alt-tab behavior on local sw600dp device.
Change-Id: Idb6f30a370192be2490c74c04d9ea81f223ecfb1