Add support for injecting RPC binder accessors to libbinder

This allows libbinder to set up client connections to binder RPC
services underneath the libbinder service manager APIs.

It requires callbacks to be added to the local process to get connection
information that the client is responsible for obtaining.
Once these callbacks are added to libbinder, any client elswhere in the
process using the service manager APIs will be able to get and use a
binder for the service in the same way they do for kernel binder
services.

Test: atest binderRpcTest vm_accessor_test
Bug: 358427181
Change-Id: Iec27d30a669e0673bd66c99fded1edc335f7dff1