Replace smart reply boolean setting with key-value list

This patch replaces the recently introduced
Settings.Global.ENABLE_SMART_REPLIES_IN_NOTIFICATIONS boolean setting
with a new Settings.Global.SMART_REPLIES_IN_NOTIFICATIONS_FLAGS
key-value list.

Rationale: This will allow us to add and tweak smart reply parameters
without polluting the global settings namespace.

Bug: 67765414
Test: atest SmartReplyConstantsTest
Change-Id: I284bb6b31618a234c4772d16ad6190a713035f1b