libbinder: Remove dependency on openssl for Trusty kernel

This modifies the trusty-specific OS.cpp to use the RNG
available in the trusty kernel instead of openssl when
TRUSTY_KERNEL_FD is defined.

Bug: 230134581
Test: build Trusty
Change-Id: Ic27406707a515882ca6340d6e8db81f3e2f9509d