Parse EXTRA_BACKGROUND_IMAGE_URI from String

It looks like it should be a Uri, but it's actually just a flat string.
Also updated the documentation to warn about this particular format of
bear trap and added a small test for the two non-message cases that we
have at the moment.

Test: atest frameworks/base/services/tests/uiservicestests/src/com/android/server/notification
Bug: 78689601
Change-Id: I50171451194bb725f31e7d6f2d1e8f0888b0a2aa