##### 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: there are invalid (non-ARN) entries in the KMS entry. These don't count as a change, but will be removed if any changes are being made.
  type: bool
  returned: always
'''
statement_label = {
    'role': 'Allow use of the key',
    'role grant': 'Allow attachment of persistent resources',
    'admin': 'Allow access for Key Administrators'
}
from ansible.module_utils.aws.core import AnsibleAWSModule, is_boto3_error_code
from ansible.module_utils.ec2 import ec2_argument_spec
from ansible.module_utils.ec2 import AWSRetry, camel_dict_to_snake_dict
from ansible.module_utils.ec2 import boto3_tag_list_to_ansible_dict, ansible_dict_to_boto3_tag_list
from ansible.module_utils.ec2 import compare_aws_tags
from ansible.module_utils.six import string_types
import json
try:
    import botocore
except ImportError:
    pass  
@AWSRetry.backoff(tries=5, delay=5, backoff=2.0)
def get_iam_roles_with_backoff(connection):
    paginator = connection.get_paginator('list_roles')
    return paginator.paginate().build_full_result()
@AWSRetry.backoff(tries=5, delay=5, backoff=2.0)
def get_kms_keys_with_backoff(connection):
    paginator = connection.get_paginator('list_keys')
    return paginator.paginate().build_full_result()
@AWSRetry.backoff(tries=5, delay=5, backoff=2.0)
def get_kms_aliases_with_backoff(connection):