gsi_util: adds sepolicy_checker

The sepolicy checker merges SEPolicy files from /system and
/vendor, /odm as the init process does when the device boots a GSI.
When the merge failed in init, the device will refuse to boot
Android OS and will jump to fastboot mode directly. It needs serial log
for diagnosis.

The action of SEPolicy merge can be done on host side, to do a trial
merge, prior to flash a GSI. The newly added sepolicy_checker is
designed for this.

Bug: 72069105
Test: m gsi_util
Test: gsi_util list_checks, output is 'vintf', 'sepolicy'
Test: gsi_util --debug check_compat --system $OUT/system.img --vendor adb sepolicy

Change-Id: I842067e447b96e2ba124f03f51831cd289f47dc1