DeskClock: Fix NPE when removing a timer

If a running timer is removed, its id might still be in TimerAdapter's
mHolders map, although it no longer exists in the DataModel. In that
case TimerFragment$TimeUpdateRunnable.run can cause
TimerViewHolder.updateTime to obtain a null Timer, and then let
TimerItem.update run into a NullPointerException when calling
getRemainingTime on that Timer.

Fixes: https://gitlab.com/LineageOS/issues/android/-/issues/8476
Change-Id: I1c002621723f972ac075ce999574a496cfbbe54d