sign_target_files_apks: adding --allow_gsi_debug_sepolicy

https://android-review.googlesource.com/q/topic:gsi_debug_policy
adds userdebug_plat_sepolicy.cil into the GSI system.img to
reduce the steps of repacking a debug ramdisk.

This CL checks that the file userdebug_plat_sepolicy.cil shouldn't
exist before signing, unless the caller explicitly specifies
--allow_gsi_debug_sepolicy to allow it.

Note: also fixes the indentation around the block.

Bug: 201482141
Test: sign_target_files_apks *-target_files-*.zip signed.zip
Change-Id: I56ed328a9ae70cf49dbd3c6efb5a4a8c54e1b7a7