Rename PREFERRED_DATA_SUBID_CHANGE to ACTIVE_DATA_SUB_CHANGE

Redefine a PhoneStateListener event so that it reflects which
subscription is currently being used for Internet data. It could
be default user data subscription or currently active opportunistic
subscription.

Bug: 124260187
Test: unittest
Change-Id: Icb90b5bd93845c29157cafddcd53335728638ec2