Fix bug 2084122: enable application control over text synth volume

This CL unhides two "keys" an application can use to specify for
 an utterance its playback volume and pan. Those two new keys
 use the same mechanism already in place for an application to
 specify stream type and utterance ID.

Change-Id: I020363487d7a0f471fb8d3ed739c561d2ab4b0a9