Define VcnStatusCallback#onVcnStatusChanged.

This CL defines VcnStatusCallback#onVcnStatusChanged for notifying
VcnStatusCallbacks when the status of their specified subscription group
changes. Specifically, status codes will be provided to notify callbacks
when a VCN is started, stopped, enters Safe Mode, and when there are no
matching VCNs.

Bug: 180659281
Test: atest FrameworksVcnTests
Change-Id: Icf686c3da6c5bdeeab88f60495d2ad438fa15692