I'm not sure how it works on an Android keyboard, but for half letters you have to use the 'halant', which looks like this ् and maps to the 'x' on my keyboard. क + ् + य = क्य
Thank you so much for this tip zeebo7, now I can finally type words like "why" or "what" in Hindi. This definitely deserves a lingot! :-)
You can download Google Indic keyboard from Play Store. It works perfectly. You type in Latin alphabet and it converts that to devnagri.
Use Gboard and you can type hindi with roman, you write kyon and he'll guess what you mean