[SB] Make OngoingActivityChipModel.Shown a sealed class.

We've been thinking of this model as a sealed class but I realized I
forgot to label it as such. This makes the update and also updates any
`when` clauses that now need to be exhaustive.

Bug: 392895330
Flag: EXEMPT refactor
Test: compiles
Test: atest OngoingActivityChipsViewModelTest
Change-Id: I7589f4c4017519b723cda1a569fdb1853e7b2d22