bug 943138 - improve dialer ADSR envelope

Scheduling tones at AudioContext.currentTime resulted in the first part of the
tone being cut off, so start an AudioBufferSourceNode for the envelope.

Add a release phase for long dialer tones.

Revert the release period to 0.05 to match the prior mozWriteAudio
implementation.
Merge pull request #14546 from karlt/943138

bug 943138 - improve dialer ADSR envelope r=etienne