This makes the following changes:

1.  setVoice takes a Voice object instead of a language string.

    This is to make the API cleaner and allows SpeechSynthesis
    to pass the correct parameter (identifier for SetVoiceByName,
    or name for SetVoiceByProperties) to espeak.

2.  The espeak_SetVoiceByName API is also exposed to support passing
    the voice variant to use.

3.  The age parameter has been re-added with the constants:
    *  SpeechSynthesis.AGE_ANY
    *  SpeechSynthesis.AGE_YOUNG
    *  SpeechSynthesis.AGE_OLD
    based on the behaviour of the eSpeak voice selection algorithm.

NOTE: Due to the way that voice selection is implemented in eSpeak,
if variant is specified, the age and gender cannot be specified and
vice versa.

This renames `setVoiceByProperties` to the more consise `setVoice`
and removes the name parameter as this is not used -- the voice is
set by the language and gender parameters only.

The `age` parameter is not useful as eSpeak does not use this when
selecting voices. This is always set to `0` by TtsService.java.

The `variant` parameter is not used to select the actual voice
variant (from the `!v` directory) but to select the n^th matching
voice from the list of matching voices. This is always set to
select the first matching voice by TtsService.java.

The sampleRate variable gets overwritten with the actual sample
rate during creation from the espeak_Initialize method, so the
value it is set to in the constructor does not matter.