Support speech recognizer confidence values.

Pass speech recognizer confidence values in the SpeechRecognizer and
RecognizerIntent APIs through to the caller. This change defines new
API constants for keys to these values.

The corresponding change is being made to Google Voice Search, and should be
made to any other implementations.

Bug: 3267513
Bug: 4163206

Change-Id: I294553f2eb9eb3be21298b8434117c8c5309558d