Use clean identity for all calls in/out of VcnMgmtSvc.

This CL updates VcnManager and VcnManagementService to use a clean
calling identify for handling all code paths entering and leaving the
System Server.

Bug: 180451994
Bug: 182183302
Test: atest FrameworksVcnTests
Change-Id: I147408544531927ecaf5e40af6cb139342f5cd63