Update Notifications sample - prep work for Kotlin version

Based on the feedback in ag/2799773 from benweiss@.

Bug: 65631986
Test: Manually ran app on emulated phone and watch, verified that there
are no regressions

Change-Id: I4b2bd955e6eb3325b76e7b36669bbab6eaba68f6
(cherry picked from commit 2f0f033a)