data usage chart: move color values to resource file

This will allow themes to specify custom colors for
the data usage chart.

Change-Id: I17669348e8ac6dce63100604c1044f9de78056c2
Signed-off-by: Dirk Rettschlag <dirk.rettschlag@gmail.com>

Conflicts:
	res/layout/data_usage_chart.xml