Creates a combined split-sepolicy file in merge_target_files.py.

This follows the same steps as OpenSplitPolicy() in
system/core/init/selinux.cpp on the device.

Bug: 178864050
Test: merge_target_files for R+S and S+S devices
Test: test_merge_target_files
Change-Id: Ia41a436bfda8e2cb65706122f0ff3805b99d16e1