Add experiment flag for smart suggestions in heads-up notifications.

With this CL we add a new key-value pair to
smart_replies_in_notifications_flags to control whether smart
suggestions should be turned on for heads-up notifications. By default
the suggestions are turned ON.

Bug: 122351821
Test: atest SmartReplyConstantsTest
Test: Run "adb shell settings put global
smart_replies_in_notifications_flags show_in_heads_up=false" and ensure
heads-up notifications do not contain smart suggestions.
Change-Id: I84292c10a4545112b0e993af33bb01a8cc18ce23