AudioService: add STREAM_DTMF to stream affected by ringer mode

-STREAM_DTMF volume muted when phone in vibration or silence
 mode but will not restore when back to normal mode by setting
 STREAM_RING volume to 1
-Add STREAM_DTMF to the list of stream volume affected by ringer
 mode

Change-Id: Ib91d6b2c09e10b5cd86ae192cf334c69884da294
CRs-Fixed: 549908