The idea here is that whenever the user changes one of the parameter
values via the SeekBar control, it is reflected immediately by eSpeak.
This is similar to how e.g. the brightness preference UI works.
However, instead of updating every time the value changes while the
slider is being moved, it is updated once the touch interaction is
finished.

Also, if the user Cancels or exits the dialog with the back button,
the old setting value is restored.

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 TTS engine should be destroyed in the onStop method, not the
onDestroy method to prevent it being used in the stopped state.

When using assistive technology like TalkBack and not accessing
either the eSpeak application or the Android TTS settings
directly, the voice data would previously not be installed/upgraded
correctly.

The approach here is to move the logic to onIsLanguageAvailable so
that if the espeak voice data is not installed or is upgradable
then trigger an install of the voice data and return
TextToSpeech.LANG_MISSING_DATA.