DeskClock: Fix left button display

* When adding a new timer, the left FAB will change to a "delete" icon
* Switching to another tab and returning will not display it anymore,
  resulting in the requirement to actually start a new timer to be able
  to access already running ones

* Test:
- Default FAB visibility is GONE until a valid value is inserted
- Changing from and to tab will display it again if value was already
  inserted
- Start timer
- Click "Add" (right button)
- Delete button will appear
- Delete button will be visible after swicthing tab and returning

Change-Id: I311faa96ea9e0320c630fa858227f2ad4625eb4e