[View binding] Add ContentDescription as part of the Icon viewmodel.

This mimics how Compose treats icons, and reminds future users of `Icon`
to add content descriptions (or choicefully not have one).

Bug: 238425913
Test: tests in systemui.qs.footer
Test: tests in systemui.statusbar.pipeline
Test: Turn on flag 507 and ensure QS footer actions still have content
descriptions

Change-Id: Ia86e50f43c36325ef7523b86dc4a8ca9b269325e