[SB][Screen Chips] Add top-level chips VM and two individual VMs.

This CL adds a top-level OngoingActivityChipsViewModel that's
responsible for determining which ongoing activity chip to show (needed
if multiple activities are ongoing at the same time).

It also adds a new interface, OngoingActivityChipViewModel (singular
"chip"), which lets each individual ongoing activity define a view model
specifying when the activity is active or not. The top-level view model
takes in all the individual view models.

Bug: 332662551
Test: All tests in statusbar.chips directory, and
CollapsedStatusBarFragmentTest
Flag: com.android.systemui.status_bar_screen_sharing_chips

Change-Id: I1995ac9ca02f89d62065c12aa5f31708a5bd5aca