[SingleLineView] Use CachingIconView for single line views

We want to have a consistent conversation layout xml structure. For conversation icon, HybridView is using ImageView while other conversation layouts are using CachingIconView.

Replacing ImageView with CachingIconView and using the same `scaleType` will make conversation icon usage consistent between conversation layouts (which will be helpful for updating templates for visual redesign).

Using CachingIconView for hybrid conversation will not have any visual difference and any memory optimization (since Hybrid views are not reused between updates).

Bug: 379253524
Test: Post grouped conversation notifications with notify and see icon rendering.
Flag: EXEMPT trivial view change.
Change-Id: I83ff56f3b425ed53eb0d2150542f701c3a239994