Focus-android: Improve/polish notification

[pocmo]

This is how our notification currently looks like (with actions from #826). It looks rather dull and monotonous.

We could at least try to add a highlight color (see follow-up screenshots). Should we also update the wording?

We started with mimicking the Chrome incognito notification - but this one does not feel modern either.

[pocmo]

For comparison some other notifications from my device:

[pocmo]

Using the FAB color for the notification:

[pocmo]

On Android O adding the launcher icon looks interesting too.

However it heavily depends on the system and Android version how the notification will actually look like (as long as we do not define a custom layout). On my Android 5.1 device the notification looks different and the color is used differently too.

[antlam]

looks good!