Load selinux compat file if it exists

This addresses Treble backwards compat issues introduced in
aosp/793958 and aosp/783669.

Bug: 122874820
Test: build/flash blueline with pi-dev vendor and generic_ab system
    images.
Test: adb pull /sys/fs/selinux/policy;
    sesearch policy --allowx -s vendordomain -t dev_type

Change-Id: I9123aa02cf4bf07d72d1ea086dde0d1cf1b30db4