Make notification match the mocks.

* Now it is of nice blue colour, lighter in night theme.
* Uses suitcase icon instead of warning sign.
* Shows "Work profile" instead of "Android system" as the source.

note: I reused a string for "Work profile", which has the same
content, but different purpose. This is not ideal, but we are way
past the deadline.

Bug: 155612405
Test: manual, with TestDPC
Change-Id: I8298401742085b1738de384e3fe0e612a8142607