Specify if a VCN is in 'test-mode' in VcnContext.

This CL updates VcnContext to include whether the VCN instance
it is created for is in test-mode (as specified by the VcnConfig).
This also changes the lifetime of VcnContext to be created uniquely
for each VCN instance (as opposed to a single VcnContext created in
VcnManagementService and used for all VCNs).

Bug: 182291467
Test: atest FrameworksVcnTests CtsVcnTestCases
Change-Id: I6fc6a266bf67ab2aa64202153c3c109ee98a16ca