Cap minimum visibilty of floating windows by the real size.

Without the cap, say, a 40dp window cannot snap to the left edge,
and one CTS is failing due to this in freeform mode.

BUG=30037893
TEST=run cts --class android.widget.cts.PopupWindowTest \
      --method testUpdateDimensionAndAlignAnchorViewWithOffsets

Change-Id: I474ca4ce519bba39d3e16ec8620b1176a3c52834