kernel: exempt from vendor_file restrictions

The kernel is unusual in that it's both a core process, but vendor
provided. Exempt it from the restriction against accessing files from
on /vendor. Also, rework the neverallow rule so that it disallows
opening/modifying files, but allows reading files passed over IPC.

Bug: 68213100
Test: build (this is a build-time test)
Change-Id: I2f6b2698ec45d2e8480dc1de47bf12b9b53c4446