Rename NotificationEntry.smartReplies to systemGeneratedSmartReplies

NotificationEntry contains smart replies and actions generated by the
NAS, so they will always be 'system generated' rather than app
generated. With this CL we make this more clear by renaming the
smartReplies field.

Test: make && atest SystemUITests
Change-Id: Ide9a4cad3847058f218b32a5049f740ab4f5f4a4