Add unknown state in DataSubscriptionState enum

Address ag/11439257 leftover comment

This change adds an unknown state in DataSubscriptionState
enum in order to pass server side compatability warning.

Besides, this change also address ag/11439257 leftover comment.

Test: build
Bug: 148905838
Bug: 129082217
Change-Id: Ib831832f2a24cf4c5ca64b078998fd93fed3793e