##### SUMMARY

If you attempt to modify the policy on an existing KMS CMK using aws_kms's "policy" option this doesn't appear to get updated.

##### ISSUE TYPE
- Bug Report

##### COMPONENT NAME
lib/ansible/modules/cloud/amazon/aws_kms.py

##### ANSIBLE VERSION

```
ansible 2.8.1
  config file = /home/mchappel/.ansible.cfg
  configured module search path = [u'/home/mchappel/.ansible/plugins/modules', u'/usr/share/ansible/plugins/modules']
  ansible python module location = /usr/lib/python2.7/site-packages/ansible
  executable location = /usr/bin/ansible
  python version = 2.7.5 (default, Jun 11 2019, 12:19:05) [GCC 4.8.5 20150623 (Red Hat 4.8.5-36)]
```

##### CONFIGURATION

```
DEFAULT_CALLBACK_WHITELIST(/home/mchappel/.ansible.cfg) = [u'timer', u'profile_tasks']
HOST_KEY_CHECKING(/home/mchappel/.ansible.cfg) = False
RETRY_FILES_ENABLED(/home/mchappel/.ansible.cfg) = False
```

##### OS / ENVIRONMENT
RHEL7 + AWS


##### STEPS TO REPRODUCE

- Create KMS CMK
- Attempt to update policy using the 'policy' option on aws_kms

```
    - name: 'Update policy on KMS Keys'
      aws_kms:
        key_id: '{{ kms_key.id }}'
        alias: '{{ kms_key.alias | default(omit) }}'
        policy: ' {{ kms_current_policy | to_json }}'
```

##### EXPECTED RESULTS
KMS Policy is updated

##### ACTUAL RESULTS
KMS Policy stays the same
description = module.params.get('description')
    
    
    if description and key['description'] != description:
        try:
            connection.update_key_description(KeyId=key['key_id'], Description=description)
            changed = True
        except (botocore.exceptions.ClientError, botocore.exceptions.BotoCoreError) as e:
            module.fail_json_aws(e, msg="Failed to update key description")
    desired_tags = module.params.get('tags')
    to_add, to_remove = compare_aws_tags(key['tags'], desired_tags,
                                         module.params.get('purge_tags'))
    if to_remove:
        try:
            connection.untag_resource(KeyId=key['key_id'], TagKeys=to_remove)
            changed = True
        except (botocore.exceptions.ClientError, botocore.exceptions.BotoCoreError) as e:
            module.fail_json_aws(e, msg="Unable to remove or update tag")
    if to_add:
        try:
            connection.tag_resource(KeyId=key['key_id'],
                                    Tags=[{'TagKey': tag_key, 'TagValue': desired_tags[tag_key]}
                                          for tag_key in to_add])
            changed = True
        except (botocore.exceptions.ClientError, botocore.exceptions.BotoCoreError) as e:
            module.fail_json_aws(e, msg="Unable to add tag to key")
    desired_grants = module.params.get('grants')
    existing_grants = key['grants']
    to_add, to_remove = compare_grants(existing_grants, desired_grants,
                                       module.params.get('purge_grants'))