[voicecall] Shorten response time when playing single DTMF tone

Signed-off-by: Martti Piirainen <martti.piirainen@oss.tieto.com>

When the user rapidly enters single DTMF digits during a voice call, oFono responds quite slowly (and some tones are therefore skipped). oFono core uses a one-second-resolution timer for handling pauses (the 'p' character), but this unnecessarily slows down the "pause length = zero" case in the queue processing. Use millisecond-resolution timer for that case.