Add close button in conversation notifications

This CL adds close/dismiss button in the upper right corner of conversation notifications. This feature can be enabled through config, and will be most useful for mouse and touchpad users.

Bug: 344681642
Test: manual with Notify1

Change-Id: Ia297951fd4eb8a2568bef92d138ea4ef1e1e4d85
(cherry picked from commit 7c2eb2dbbfcfd44056b8820de23d8c341bdd0312)