Make it possible to specify a list of "verbatim files"

This allows us to exclude several files from the patching algorithm, which is required because some files in /system are changed by flashing gapps (namely: system/app/Provision.apk).
Example usage:
```build/tools/releasetools/ota_from_target_files --no_separate_recovery true --device_specific device/moto/jordan-common/releasetools/jordan-common_ota_from_target_files.py --verbatim_files system/app/Provision.apk --incremental_from cm_mb526-target_files-old.zip cm_mb526-target_files-new.zip incremental-update.zip```