Google DialogFlow will use cloud compute to handle user request, so that I can process huge text input and throw to a certain INTENT(Android INTENT)
What can I response to let google assistant uses slice fulfillment defined in actions.xml
At this stage (Sept 2019), the implementation of an Assistant Custom Conversation (for which DialogFlow is one option for fulfilment) is completely separate to App Actions (which uses actions.xml inside your Android app code).
Slices are part of the App Actions implementation and are therefore not related to Assistant Custom Conversations.