Prevent concurrent modification exceptions in VcnManagementService

In order to prevent concurrent modifications from triggering an
exception in the dump, proxy it to the handler thread.

Adding locks was considered, but would require auditing all callbacks
and proxying them to the handler threads in order to ensure lock
inversion and deadlocks never occur.

Bug: 188840899
Test: FrameworksVcnTests
Test: adb shell dumpsys vcn_management
Change-Id: Ibcc4bdc06301dd77260adaa0c086529f8f524679