[SB][Screen Chips] Show 3-2-1 countdown for screen record chip.

This updates the OngoingActivityChipModel.Shown to have two variants:
 - Timer: For chips that show a timer counting up
 - Countdown: For the 3-2-1 countdown
The model now allows for a null icon and a null click listener, since
the countdown chip has neither.

This also adds a new TextView to the chip view, used for the countdown.

This also updates the click listener for the call chip to be null if
there's no intent, now that we support a null click listener.

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

Test: Verify old ongoing call chip with the flag off still has the right
spacing
Test: Start screen recording -> verify chip shows the 3-2-1 countdown,
then switches to the timer chip counting up
Test: Verify all new chips still work with icons and timers
Test: Increase font size so that there's not enough room for the timer
on the chip -> verify only the icon shows and the icon is centered

Change-Id: I358233473dd098ad23034a6b41876a7bfa67a8a0