New device policy API: DeviceAdminReceiver.onUnsafeOperationStateChanged()

This API can be used by DPCs to re-try operations that failed due to
UnsafeOperationExceptions.

Test: m update-api doc-comment-check-docs
Test: manual verification (CTS tests will be added next) with
      adb shell cmd device_policy set-operation-safe 1 1 && \
      adb shell cmd device_policy is-operation-safe 1
Bug: 178494483

Change-Id: Ia0e55b509a4193d7c3714bbffbaa6489258af270