Add exp flag for min number of sys gen smart replies in notifications

For system (assistant) generated smart replies we want to ensure the
replies don't seem biased (see example [1]). Therefore we here add an
experiment flag that determines the minimum number of replies N to show
in a notification. If we can't fit N replies into the notification we
remove all replies from that notification (i.e. we show zero replies).

[1] Example of biased replies:
Message: "Hi, how are you?"
Replies: "Good", "Great!", "Bad"
using only one reply here ("Good") might be seen as biased.

Bug: 120779592
Test: atest SystemUITests
Test: call "adb shell settings put global
smart_replies_in_notifications_flags enabled=true,min_num_replies=X" for
different values of X and ensure the behaviour is correct.
Change-Id: I898850f7895d0fd43ec6788095162f3138cd62cb