[ROSP] Add framework for status bar popups

This CL introduces the basic framework for status bar popup chips.
It adds a media control popup that appears when the corresponding
chip is toggled.

Specifically:
- Updates `PopupChipModel` to hold a `popupViewModel`.
- Adds `StatusBarPopup` composable to render the popup based on the
  `popupViewModel`.
- `StatusBarPopupChipsViewModel` tracks the currently selected chip and handles chip toggling.
- `StatusBarPopupChipsContainer` renders `StatusBarPopup` if a chip is toggled.

The actual UI of the media popup will be added in a follow-up CL.

Bug: b/394887130
Flag: com.android.systemui.status_bar_popup_chips
Test: Unit test in StatusBarPopupChipsViewModelTest
Test: Show a prototype MediaCarousel in `StatusBarPopup`

Change-Id: I9785406072dc09aa8601576940c3076fa9e2cf62