Avoid showing timer creation inappropriately

Bug: 27360981

When no timers exist the timer creation screen is displayed by default.
If a new timer is created via an Intent (e.g. GSA), the timer creation state
was sticking around. When TimerFragment was next onResume()d, the
timer creation screen was incorrectly brought forward to honor the
stale timer creation state.

The stale state is now cleared each time the timer creation screen is
torn down.

Change-Id: I9122943c2f8521af040153bcc5ba647b35f0346f