fix scrollY when the GridView has vertical spacing

[jaredrummler]

The scrollY is off when items have vertical spacing. This fixes the scrollY jumping to incorrect values while scrolling.

This will still be an issue on pre-JB. If you did want to fix this problem for API 11-16, you could use reflection to get the vertical spacing. Example:

private Integer mVerticalSpacing;

@Override public int getVerticalSpacing() {
  if (VERSION.SDK_INT >= VERSION_CODES.JELLY_BEAN) {
    return super.getVerticalSpacing();
  }
  if (mVerticalSpacing == null) {
    try {
      Field field = GridView.class.getDeclaredField("mVerticalSpacing");
      if (!field.isAccessible()) {
        field.setAccessible(true);
      }
      mVerticalSpacing = field.getInt(this);
    } catch (Exception e) {
      mVerticalSpacing = 0;
    }
  }
  return mVerticalSpacing;
}

[coveralls]

Coverage increased (+0.02%) to 93.384% when pulling 3abb050 on jaredrummler:master into 47a5fb2 on ksoichiro:master.