Fix bug where font size (etc.) doesn't change after changing display settings.

The metrics we get from Display#getMetrics does not change regardless of
what the new configuration is (we would expect the scaledDensity to change).

Documentation shows that Display#getMetrics is deprecated in R.

Using guidance from b/154665987#comment14 we get the metrics from the derived
context.

Bug: 156141463
Change-Id: I25e5f2c13f94e0471111f6c895694947998e3222