FIX 1839556 -- Silent mode not really silent

Fix the dialer to not play touch tone sounds if the phone is in "silent mode".

Note we have to re-check for silent mode *every time* the user presses a
key, rather than just in onResume(), since the user can toggle silent mode
via ENDCALL-longpress (without actually leaving the current activity.)

TESTED:
  - "Audible touch tones" preference off: no tones
  - "Audible touch tones" preference on, silent mode off: tones play
  - "Audible touch tones" preference on, silent mode ON: no tones
  - Switch in and out of silent mode via ENDCALL-longpress, confirm change
    takes effect immediately.