Notify VcnStatusCallbacks for VCN start/stop.

This CL updates VcnManagementService to notify VcnStatusCallbacks when a
VCN is started or stopped. VcnStatusCallbacks will also be notified for
config changes that cause a VCN to exit Safe Mode.

Bug: 180659281
Test: atest FrameworksVcnTests
Change-Id: I4168c868185880621333855dfcb51e46cb662741