Fix enrollment cancelation race conditions.

This applies the same request/cancel mechanism as commit bdd477a5.

Refactor the scheduler into smaller components and add missing test coverage.

Fix: 210090299
Bug: 208303000
Test: atest UserAwareBiometricSchedulerTest BiometricSchedulerTest BiometricSchedulerOperationTest  SensorTest Face10Test
Test: manual (enroll and quickly tap "add another" and then "do it later")
Change-Id: I0c7e4006e6ac6e6c3dc115046fa80d9e60f4bd95