Add internal as a new permission protection level.

Permissions with the new internal protection level are similar to
signature permissions in that they can be controlled internally by the
system via the specified protection flags, but they are no longer
granted according to signatures. This enables us to create role-only
permissions.

Bug: 158736025
Test: presubmit
Change-Id: Ie5f76f14d6e32b05b9113090e10b8a1a41a2d9da