Fix live timers flickering when pausing

This change adds a flag which uses a stable id as the key for timers,
instead of using the smartspace target id. The stable id stays the same
when pausing/playing timers, which allows us to re-use the existing
AndroidView of the timer instead of creating a new one.

Bug: 353801573
Test: atest SystemUiRoboTests
Flag: com.android.systemui.communal_timer_flicker_fix
Change-Id: I1124a9148ddd635899eb5d9f6780cd49add9f232