Revert "Revert "Allow other processes to implement a UDS service..."

Revert submission 33153434-revert-32747642-avf_vm_client_host_service-OVCFKYECGS

Reason for revert: Fixing issues.

1) servicemanager not handling host tests with kernel binder enabled without the kernel binder driver loaded. Need to support this in BackendUnifiedServiceManager
2) microdroid_mgr forking and the new forked processes have the rpc_servicemanager FD that they don't have SELinux permissions to have. We need to get the FD with FD_CLOEXEC before microdroid_manager forks anything.

Reverted changes: /q/submissionid:33153434-revert-32747642-avf_vm_client_host_service-OVCFKYECGS

Flag: EXEMPT Clients of this new functionality will be flagged
Test: atest vm_accessor_test MicrodroidHostTestCases
Bug: 358427181
Change-Id: Idfdb711975889098e55ef61a130008a3a0d1c0e9