Limit the systemconfig tags allowed to vendors

Vendors are allowed to customize these systemconfig tags only.

<library>
<feature>
<privapp-permissions>

This is because the systemconfig tags are essentially the part of system
<-> vendor interface and thus need to be stable (or evolve in a backward
compatible manner) across several Android releases and we would like to
keep the interface as small as as possible.

However, since vendors were allowed to use more tags (like <permission>,
<app-link>, <default-enabled-vr-app>) in Oreo and Oreo-MR1, this
limitation is applied only for newly launching devices whose first API
level is equal to or greater than P.

Bug: 70821981
Test: wahoo is bootable (launched with Oreo)
Test: crosshatch is bootable (launched with P)
Test: adb logcat -s SystemConfig does not show that a tag is not
supported

Change-Id: I371b93a80f3d9728ea6d35022081776a8658d9f3