set KEEP_CONNECTED_FOR_HANDOVER for IWLAN HO

- Update NetworkScore with KEEP_CONNECTED_FOR_HANDOVER to indicate
handover is in-progress.
- Legacy score has not been used to compute Network's reassignment at Connecitivity module, so removed legacy score update from DataNetwork.

Bug: 275470915
Test: FrameworksTelephonyTests
Change-Id: I643cd609838b43c6ef5e84c42da560c587bbbe0d