Allow other processes to implement a UDS servicemanager

Another process can serve IServiceManager over
/dev/socket/rpc_servicemanager.

Once the servicemanager.ready property is set, IServiceManager clients
will look for the kernel binder servicemanager if kernel binder is
supported, and fall back to checking for the UDS servicemanager.

This is useful for systems without kernel binder, with unix domain
sockets, to provide the same experience for binder clients trough the
IServiceManager client APIs.

Flag: EXEMPT Clients of this new functionality will be flagged
Test: atest vm_accessor_test
Bug: 358427181
Change-Id: I78e85fb62c67f6b7a085fda9a681e569bf32ed01