libbinder: Add getter for max supported FDs

Add a getRpcTransportModeMaxFds constant getter that
returns how many file descriptors are supported by
the current Rpc file descriptor transport mode.
This is in preparation for enabling file descriptor
support in the Trusty tests.

Bug: 242940548
Test: m binderRpcTest
Change-Id: Ia80fa0e992e2db6f29fef8f1361a208327a4ab37