Prevent non-system apps from read ro.usb.uvc.enabled

ro.us.uvc.enabled should not be readable from apps that can't or
shouldn't act on UVC support. This means all non-system apps. This CL
adds an explicit neverallow rule to prevent all appdomains (except
system_app and device_as_webcam).

Bug: 242344221
Bug: 242344229
Test: Build passes, manually confirmed that non-system apps cannot
      access the property
Change-Id: I1a40c3c3cb10cebfc9ddb791a06f26fcc9342ed9

private/property.te

@@ -687,3 +687,10 @@ neverallow {
   -init
   -vendor_init
 } usb_uvc_enabled_prop:property_service set;
+
+# Disallow non system apps from reading ro.usb.uvc.enabled
+neverallow {
+  appdomain
+  -system_app
+  -device_as_webcam
+} usb_uvc_enabled_prop:file no_rw_file_perms;