libbinder: Add internal API to RpcServerTrusty for Rust

Add an internal API for the tipc connection handlers
that we can plug into the Rust service implementation.

Bug: 242243245
Test: m libbinder_on_trusty_mock
Test: trusty/vendor/google/aosp/scripts/build.py \
    --test "boot-test:com.android.trusty.binderRpcTest" \
    qemu-generic-arm64-test-debug
Change-Id: I667c2f8dfc495ece5f9b1309b124737e30e82909