libbinder: move session ID RNG code to Utils.cpp

This moves the code that reads from /dev/urandom from RpcServer.cpp
into Utils.cpp so other operating systems can provide their own
implementations by replacing Utils.cpp.

Test: atest binderRpcTest
Bug: 224644083
Change-Id: I2923b25537c07060b830b0d8378df8c969bbd02f
3 files changed