Add utility method for NotificationContentViewTest setup.

There's a lot of code in NotificationContentViewTest that's used over
and over again in different test cases - with this CL we merge this code
into a single method.

Bug: 120842685
Test: atest NotificationContentViewTest
Change-Id: Iab1db040ac8050e096c4ad375bc1a7dacc28b8e1