Change 640 integer to DisplayMetrics.DENSITY_XXXHIGH in
getLauncherLargeIconDensity method.

Now DENSITY_XXXHIGH constant value is in DisplayMetrics.
DisplayMetrics.DENSITY_XXXHIGH is more clear than
DisplayMetrics.DENSITY_XHIGH x 2

Change-Id: Iebdc7634171d4caca7f57ed101205e80525e26ea