If you want to stop poking in the dark, you need to clarify, whether the problem is on the arduino side or on the android side. If you are not sure about this, you will waste your time, and ours.
So - forget the arduino for now. Connect your PC via bluetooth to your android. Send serial messages from the PC serial terminal app to the android serial app (NOT the TTS app).
Does this work?
If no, get your settings on the terminal apps right. This MUST work.
If serial comminication works: send these serial messages to the TTS app. Does this work?
If no, there could be several reasons: TTS app maybe expects newline or return at the end of every word? Maybe your android text to speech engine is not working (this is independent of the TTS app!)? Try another TTS app to verify this.
If yes: then you at least know that the TTS app works in principle. Now there must be a difference between the type of message from your PC (they work) and the type of message from your arduino (they don‘t work). Compare the serial messages, look at hidden characters, etc.
Contact the author of the TTS app.
Okay, to be honest with you: You keep saying that it doesn‘t work, but you are not trying systematically to find the problem. It‘s your job now. Give us the answers to all of the above tests, if you want more help.
Thomas