Adding gatekeeperd.h and moving declarations for GateKeeperProxy into
header. Adding separate main.cpp for entry point.
Adding cc_default to be used by binary and fuzzer.
This is needed in order to fuzz gatekeeperd AIDL service
Test: m gatekeeperd
Bug: N/A
Change-Id: Ifb2082f68bb200e4ed83608a6def8e45032753de