Rename AuthenticationResult#gkResponse

Rename it to just 'response'.  It is not specific to Gatekeeper.
No functional change.

Test: atest FrameworksServicesTests:com.android.server.locksettings
Bug: 320392352
Bug: 395976735
Flag: EXEMPT refactor
Change-Id: I534608f76d4eb972a0bba03eea2e3698e2a29848