all: make unit.Converter concrete and rename to Metric

An interface for scaling dp and sp is overkill, at least for all
current uses. Make it a concrete struct type, and rename it to the
shorter and more precise Metric.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

app/internal/window/os_android.go

@@ -396,9 +396,9 @@ func (w *window) draw(sync bool) {
 				Y: int(height),
 			},
 			Insets: w.insets,
-			Config: &config{
-				pxPerDp: ppdp,
-				pxPerSp: w.fontScale * ppdp,
+			Metric: unit.Metric{
+				PxPerDp: ppdp,
+				PxPerSp: w.fontScale * ppdp,
 			},
 		},
 		Sync: sync,