Changes to icon picker for reusability in "add mode" flow

* Don't finish the fragment from the controller (ugh!) instead just report the selected icon via a listener.
* Highlight the selected icon in the list.
* Cache the icon drawables (since we're using selectors for the colors, we don't need to swap them, one per icon resource id is enough).
* Improved the tests a bit too.

Bug: 333901673
Bug: 326442408
Test: ates
Flag: android.app.modes_ui
Change-Id: Ib2ec7a7e3ed99b13f9264aa6f7c209ee3f6967a0