Fix string concatenation error

Fix an error caught by golint that prevents using go test.  string(int)
produces a rune, not a string representation of the int.  Use fmt.Errorf
instead to format the string.

Test: go test
Change-Id: I4c2e1e30a0f1b3dc24afd123c38c4e41c2abb47a

sdk/update.go

@@ -911,7 +911,7 @@ func newOsTypeSpecificInfo(ctx android.SdkMemberContext, osType android.OsType,
 
 	if commonVariants, ok := variantsByArchName["common"]; ok {
 		if len(osTypeVariants) != 1 {
-			panic("Expected to only have 1 variant when arch type is common but found " + string(len(osTypeVariants)))
+			panic(fmt.Errorf("Expected to only have 1 variant when arch type is common but found %d", len(osTypeVariants)))
 		}
 
 		// A common arch type only has one variant and its properties should be treated