fix CTS DisplayTest.testGetMetrics()

The test asserts the second display's DPI settings are what they should
be that it creates. However, DisplayInfo.getMetricsWithSize() method was
calling updateDensity() which actually resets internal members to the
primary screen's current setup (which is persisted via a build.prop
setting).

Change-Id: I589d56e98fd76f091e3105df099e5d7efd8b853e
Signed-off-by: Roman Birg <roman@cyngn.com>