Require partially offscreen apps to have 48dp empty space visible.

When a freeform task is dragged offscreen, prevent task position from
going further on the x axis if doing so would cause less than 48dp of
empty header space to be visible, or on the y axis if doing so would
cause less than 48dp + gesture nav height to be visible.

Bug: 312747430
Bug: 315333951
Test: Manual
Change-Id: I2bfb195e6e707fc43f99555bd51d59318f69685e