We here add an experiment flag to limit the number of smart actions
shown per notification (including turning all small actions off) from
the System UI.
Bug: 122506860
Test: atest SystemUITests
Test: call "adb shell settings put global
smart_replies_in_notifications_flags enabled=true,max_num_actions=X" for
different values of X and ensure the behaviour is correct.
Change-Id: I2759f4bc297386d20f3a6d1cb2975165e5c87c38