track-element: Ignore writability for whitlisted children props

If the property was explicitely whitelisted, we should expose it
in any case.

This was a regression from 835d6937
6 jobs for fix_adding_readonly_children_props in 28 minutes and 4 seconds (queued for 1 second)
Status Job ID Name Coverage
  Preparation
passed #73145
gst indent

00:00:20

passed #73144
manifest

00:00:36

 
  Build
passed #73146
build fedora x86_64

00:04:47

 
  Test
passed #73147
check fedora

00:15:14

 
  Full Builds
passed #73148
build android arm64 api28

00:04:08

passed #73149
build cerbero fedora x86_64

00:07:26