[SB][ComposeIcons] Add support for custom SystemStatusIconViewModels

The 'icon' property on SystemStatusIconViewModel is being replaced with a 'visible' property and a new 'Default' sub-interface that has an 'icon' property. This allows for more flexibility in the types of view models that can be displayed.

Test: SystemStatusIconsViewModelTest
Bug: 427976550
Flag: com.android.systemui.status_bar_system_status_icons_in_compose
Change-Id: I080eea28c489fa3f152bb61d2e0f79e19966ee7d