Fix creation of first timer via voice

Bug: 25843144

Attempting to view a specific timer trumps the timer setup screen
in all cases. onResume() now enforces that policy. The timer setup
screen is still displayed in all other cases, including:

- no timers exist
- the intent indicates a new timer should be created
- the last visible timer screen was the timer setup screen

Change-Id: I28d147cadeb29055fa0c68a44da6648f6ad33238