Split UdfpsController and backfill tests.

This splits the monolithic original class into a smaller event handler (original) that delegates UI/request state to a new overlay state holder class, which is migrated to Kotlin.

Test: atest UdfpsControllerOverlayTest UdfpsControllerTest
Test: manual (enroll manually and BP test app)
Bug: 205875955

Change-Id: I8d12a3d130694d2a3fdc1ccb44cf2d30acd92376