Fab should shrink/grow as user changes tabs.

Bug: 25948707

Note: Even though the "play" icon is routinely displayed on
the TimerFragment and StopwatchFragment they are separate
actions with entirely different outcomes, so the fab still
shrinks/grows when switching between those tabs.

Change-Id: If4f433983627175490aef64eb4e556e07e9bd87d
4 files changed