recovery: install system_ext file_contexts and property_contexts

System Ext partition is not mounted in recovery mode. Install
system_ext_file_contexts and system_ext_property_contexts into
rootdir for recovery. This is similar to how we deal with system,
product, vendor, odm *_contexts files.

Bug: 137712473
Test: m bootimage for pixel 2, system_ext_file_contexts and
      system-ext_property_contexts appear in $OUT/recovery/root/
Change-Id: I94b965d775e727fbbe26c8b715bbce3ca6179fa7