[Launcher] App grid support for reset (1/4).

This CL chain provides a rewritten implementation of the UI for the app
grid selector which supports the new wallpaper picker "reset"
functionality. A previous attempt that kept the UI's old implementation
required the introduction of many hacks in classes that are shared
across multiple selector-like experiences.

Bug: 267804479
Test: unit and integration JUnit tests added for the data, domain, and
ui layers of the wallpaper picker code
Test: manually made sure that the selector works to switch between
different launcher app icon grid options; the preview of the launcher
updates correctly; when exiting back to the main wallpaper picker
screen, the reset button appears and the section item is updated to show
the currently-selected option; touching the reset button and confirming
the reset correctly reverts to the original option, updating the section
item as well

Change-Id: If1f8c96ef5004125b61ff6b4ae19848932693e0f