Portal and Half-Life 2 are now available on the NVIDIA Shield via the Google Play store.
Your Steam account is separate from your Google Play/NVIDIA Shield account. Purchases of games made on Google Play using the NVIDIA Shield will not provide you with a copy of the game on Steam.
NVIDIA Shield account and technical support is provided by NVIDIA, including support for the Shield versions of Portal and Half-Life 2.
For Google Play purchase and account issues, please contact Google Support: