Update shortcut label for "Open assistant"

Given that users have the option to set up Google Assistant,
or another assistant service, we use "digital assistant" in
lowercase as a catch-all term in these scenarios.

Fix: 388502407
Flag: com.android.systemui.keyboard_shortcut_helper_shortcut_customizer
Test: Manual - check that string is updated in the UI
Change-Id: Ida06fec635af5cbcd531cea9bf1c213cb66c45ba