On TV, make KEYCODE_SEARCH launch Assistant with keyboard

Currently, KEYCODE_ASSIST and KEYCODE_SEARCH both launch Assistant with voice UI on TV. This change splits the behavior of the keycodes, making KEYCODE_SEARCH suitable for mic-less remotes that wish to show text input instead of voice input.

Test: manual -
adb shell input keyevent KEYCODE_SEARCH -> keyboard UI
adb shell input keyevent KEYCODE_ASSIST -> voice UI
Assistant button on paired bluetooth remote -> voice UI

Flag: NONE small, TV-only change

Bug: 315264352

Change-Id: I764cddbfe84954d4712c8efab2c4f8fa087e0fb2