Remove onWindowFocusChange solution from FPS enrollment

Context:
The legacy architecture UdfpsEnrollView was shown by SysUI
which has higher z-order that encounter lots of overlap issue.
Besides, IPC impact the latency of show/hide enroll view in
legacy architecture.
We used to upload ag/19699461 to cancel enrollment due to
above limitation.

With the new architecture ag/20582186 the UdfpsEnrollView
migrate to Settings and controlled by Settings process.

And ag/21149538 config SETTINGS_SHOW_UDFPS_ENROLL_IN_SETTINGS flag
to ture by default.

It's the time to improve the enroll UX by removing cancel
signle when onWindowFocusChanged() invoked.

Bug: 262680438
Test: m RunSettingsRoboTests -j30 ROBOTEST_FILTER=FingerprintEnrollEnrollingTest
Test: manual Security Settings > Enroll fingerprint > enroll
      Swipe Notify Shade , Heads-up, Power menu interrupt should
      not stop the enroll flow.
Change-Id: I60d88d5edd80e6eed70e2fbf288357e949abcae3