TimerFragment should only adjust the fab when it is selected

TimerFragment has a vertical pager that presents a vertical
list of all defined timers. If changes occur to that pager
(as they do in TimerFragment.onResume()), it was possible
for the TimerFragment to incorrectly adjust the fab icon
when it was not the selected tab.

Bug: 22159053
Change-Id: Ie62fbf0a2a7bee4dd69660422ff8741de6cb20c8