Add conversations image resource

- Conversations image resource is a compilation of the conversation
icons that are allowed to bypass DND
- Update category header text on DND > Display options for hidden notifications
page
- Update a11y bug in messages and calls pref pages that said images were
tappable

Test: manual
Bug: 151845457
Change-Id: I5387722abf0543b6e07378d498cc0085122c91b9