Add experiment flag for maximum number of smart actions per notification

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