Remove references to nonplat sepolicy

"nonplat" was renamed to "vendor" in Android Pie, but was retained
here for Treble compatibility.

We're now outside of the compatbility window for these devices so
it can safely be removed.

While I'm here, improve accuracy of some comments and
in-code-documentation.

Test: build boot cuttlefish device. adb remount, modify
/system/etc/selinux/plat_sepolicy_and_mapping.sha256 to force
on-device policy compilation. reboot. Verify that device boots
without new selinux denials.

Change-Id: Ibe5c5fa1ea206c1b4d5ad8183433c332a8aaadbf