[AppConfig] Fix bug in print preview of restore command

[avanigupta]

Description

There was a bug in converting KeyValue object to JSON during print preview of az appconfig kv restore command. Fixed the bug by accessing the correct property, which is called locked, instead of read_only.

Testing Guide

This is the unhandled error that will be fixed by this PR:

az appconfig kv restore -n test-appconfig-00  --datetime 2021-03-16T00:00:00Z

---------------- Preview (Beta) ----------------

Deleting:
The command failed with an unexpected error. Here is the traceback:
'KeyValue' object has no attribute 'read_only'
Traceback (most recent call last):
  File "C:\Users\VSSADM~1\AppData\Local\Temp\pip-unpacked-wheel-up6eyly2\knack\cli.py", line 233, in invoke
  File "C:\Users\VSSADM~1\AppData\Local\Temp\pip-unpacked-wheel-cg742dxv\azure\cli\core\commands\__init__.py", line 664, in execute
  File "C:\Users\VSSADM~1\AppData\Local\Temp\pip-unpacked-wheel-cg742dxv\azure\cli\core\commands\__init__.py", line 727, in _run_jobs_serially
  File "C:\Users\VSSADM~1\AppData\Local\Temp\pip-unpacked-wheel-cg742dxv\azure\cli\core\commands\__init__.py", line 720, in _run_job
  File "C:\Users\VSSADM~1\AppData\Local\Temp\pip-unpacked-wheel-c__gk16o\six.py", line 703, in reraise
  File "C:\Users\VSSADM~1\AppData\Local\Temp\pip-unpacked-wheel-cg742dxv\azure\cli\core\commands\__init__.py", line 698, in _run_job
  File "C:\Users\VSSADM~1\AppData\Local\Temp\pip-unpacked-wheel-cg742dxv\azure\cli\core\commands\__init__.py", line 331, in __call__
  File "C:\Users\VSSADM~1\AppData\Local\Temp\pip-unpacked-wheel-cg742dxv\azure\cli\core\__init__.py", line 816, in default_command_handler
  File "C:\Users\VSSADM~1\AppData\Local\Temp\pip-unpacked-wheel-4iz7mv8d\azure\cli\command_modules\appconfig\keyvalue.py", line 672, in restore_key
  File "C:\Users\VSSADM~1\AppData\Local\Temp\pip-unpacked-wheel-4iz7mv8d\azure\cli\command_modules\appconfig\_kv_helpers.py", line 664, in __print_restore_preview
  File "C:\Users\VSSADM~1\AppData\Local\Temp\pip-unpacked-wheel-4iz7mv8d\azure\cli\command_modules\appconfig\_kv_helpers.py", line 544, in __serialize_kv_list_to_comparable_json_list
AttributeError: 'KeyValue' object has no attribute 'read_only'
To open an issue, please run: 'az feedback'

History Notes

[Component Name 1] BREAKING CHANGE: az command a: Make some customer-facing breaking change.
[Component Name 2] az command b: Add some customer-facing feature.

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[avanigupta]

cc @shenmuxiaosen

[shenmuxiaosen]

[avanigupta]

Hi @jiasli / @fengzhou-msft , can you please review this PR and merge it?