Split permission: Check feature flag when SystemConfig parses xml tag

Also replaces param XmlResourceParser with XmlPullParser because the latter is a broader interface and satisfies all requirements in the calling method skipCurrentElement.

Testing: Manually on a device. Verified that split-permissions behind a disabled feature flag were absent. Override the flag in device config to enable it, rebooted, then the split-permission was present. More details in https://docs.google.com/document/d/19Yg1s1dRahnSfiRMqDukcT4XD3fD8tVZCdSRaVyKlyY/edit?tab=t.0
Bug: 364642090
Bug: 364638912
Flag: android.permission.flags.replace_body_sensor_permission_enabled
Change-Id: Ideeb989a3836b9c5695b6044cc9909ba15ed3ef4
2 files changed