Notify VcnStatusCallback#onVcnStatusChanged for Safe Mode.

This CL updates VcnManagementService to notify callbacks when VCNs enter
Safe Mode via #onVcnStatusChanged. Callbacks were previously notified
via #onEnteredSafeMode - however, VcnStatusCallbacks were changed to
provide more status information by defining onVcnStatusChanged(int) per
API Council feedback.

Bug: 1597023
Test: atest FrameworksVcnTests
Change-Id: I34ef76ec29f4369390449b70b37bc3cf99c09c77