Move SSIN sys ui flags from Settings.Global to DeviceConfig.

Migrate SmartReplyConstants flags to use the new DeviceConfig setup in
Android Q.

Bug: 123630933
Test: atest SystemUITests
Test: 'adb shell device_config put systemui ssin_max_num_actions X' for
different values of X, and ensure the corresponding max # actions are
shown in notifications.
Change-Id: I386fec12effa0153c03a328c138a0dac38bcc317