Add /system_ext sepolicy support to SELinuxMMAC.java

Bug: 137712473
Test: enable system_ext sepolicy, then boot crosshatch
Test: the following log entry is in `adb logcat -s SELinuxMMAC`
      SELinuxMMAC: Using policy file /system_ext/etc/selinux/system_ext_mac_permissions.xml

Change-Id: I684c920f8ecf2b5f29f2ad8901e187b7eae1bc89