Change timers only if no disambig necessary

Currently voice actions on timers act on a timer that's last
 on the Timers list in the database which might not be the timer
 that the user wants to change. This CL changes it so that
 a voice action on Timers is successful only if it's clear which
 Timer a user means. DELETE_TIMER only acts if there's only
 one available timer. START_TIMER only acts if there's one
 non-running timer. RESET_TIMER only acts if there's one
 stopped timer. STOP_TIMER only acts if there's only one
 non-stopped timer.
Tests are added in a separate CL.

Change-Id: Ibbd86bdd27f7936674e95e6b7966fbb72a4264a1