Ability to specify TTS engine to HERE SDK

[bhavinmdesai]

Describe the bug
Currently the voice prompts are based on the default text to speech engine of the phone. If there are multiple text to speech engines are available, it should be possible to pass the name of text to speech engine to the HERE SDK so that the SDK doesn't default to the one being used by default in the phone.

Additional context
I am expecting something similar to TextToSpeech class of Android framework where it is possible to pass the engine name.