libbinder: fix trusty cpp fd transport modes

This moves the call to set allowed file descriptor transport modes to a
common method that is invoked on both cpp and rust construction of
RpcServerTrusty objects.

Test: atest -c frameworks/native/libs/binder/tests/
Bug: 393346719
Change-Id: I0ec04676ff36879ce2b40f5070214b1cbe21f2b0