The correct buttons for SHIELD to match every button on the included 2017 remote for both short and long press, are all from the HID Consumer Usage table - and none are gamepad buttons. Flirc's Shield profile doesn't use all these codes (only some) and can't use higher numbered ones with the current firmware, so it's impossible at the moment to mimic all the buttons properly. Example: long-press of back button doesn't display power menu. Double-press home doesn't bring up task switcher. Both because incorrect keyboard codes are being used instead of the correct consumer HID codes.
Incidentally, the functionality of the MIC/Voice button on the 2017 remote can be duplicated with Consumer code 0x221
A number of keyboards, including some from Logitech and Microsoft feature keys that use these consumer codes - they work perfectly to match NVIDIA's remote buttons in every situation, regardless of app/context.