[SB][Screen Chips] Don't show time on call chip if time is invalid.

If you start an outgoing call but the other person hasn't answered yet,
the `when` value on the call notification is 0. We shouldn't show the
time in that case. This CL handles that case correctly now.

Bug: 332662551
Flag: com.android.systemui.status_bar_screen_sharing_chips

Test: trigger a call and manually set the `when` value on the call
notification to 0 -> verify chip is shown with no timer
Test: trigger a call with a valid `when` value -> verify chip is shown
with timer
Test: atest CallChipViewModelTest

Change-Id: If202eb8a54a215fca3db762d946fe13d574aa832