Allow NotificationAssistantService to suggest smart replies

If the apps has provided their own choices, they will be used, as opposed
to the "smart replies" from NAS.
Otherwise, smart replies will be applied to the notifications
with a freeform RemoteInput but without choices.

The smart reply model is not ready yet, so canned response is hardcoded
and it is disabled by default. To test it out, run
adb shell setprop persist.sys.smart_replies_experiment true
Also, to get rid of the target >= P SDK requirement, you may want to run:
adb shell settings put global smart_replies_in_notifications_flags enabled=true,max_squeeze_remeasure_attempts=3,requires_targeting_p=false

Test: atest SystemUITests
Test: atest frameworks/base/services/tests/uiservicestests/src/com/android/server/notification/NotificationListenerServiceTest.java
Test:
1. adb shell setprop persist.sys.smart_replies_experiment true
2. adb shell settings put global smart_replies_in_notifications_flags enabled=true,max_squeeze_remeasure_attempts=3,requires_targeting_p=false
3. Send a message to myself, observe the hardcoded smart replies.

Bug: 111674540

Change-Id: Ia61a77faef7c4dcba0501abfec80e3e8cc7274e4