Refactor the color span code inside Notification.java and add tests.

* This makes it easy to reduce the contrast requirement between the notification background and the button fill in another review.
* This also fixes a bug where partial-length color spans within the emphasized buttons would be contrast-adjusted only with the notification background color rather than the button fill color.

Bug: 196393060
Fixes: 197572220
Test: manual testing using the go/notify2-apk and the CallStyle color customizer
Test: android.app.NotificationTest
Change-Id: I712e6d6b2da1de89afeacda7bd2fc4ad24632811