Enabling Dialer to play ringtones

+ The code to make the Dialer play was disabled behind flags, this
change flips the flags so the Dialer is responsible for playing the
ringtone.
+ The in-code flag will be removed later after a QA pass of the
feature.
- Removed TODO to respect Do Not Disturb, see b/26936902

Bug=18702990

Change-Id: I773c1c614beb61a6763daccd2903d9d341cf4dc0