##### 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: list of grants associated with a key
  type: complex
  returned: always
  contains:
    constraints:
      description: Constraints on the encryption context that the grant allows.
        See U(https://docs.aws.amazon.com/kms/latest/APIReference/API_GrantConstraints.html) for further details
      type: dict
      returned: always
      sample:
        encryption_context_equals:
           "aws:lambda:_function_arn": "arn:aws:lambda:ap-southeast-2:012345678912:function:xyz"
    creation_date:
      description: Date of creation of the grant
      type: str
      returned: always
      sample: 2017-04-18T15:12:08+10:00
    grant_id:
      description: The unique ID for the grant
      type: str
      returned: always
      sample: abcd1234abcd1234abcd1234abcd1234abcd1234abcd1234abcd1234abcd1234
    grantee_principal:
      description: The principal that receives the grant's permissions
      type: str
      returned: always
      sample: arn:aws:sts::0123456789012:assumed-role/lambda_xyz/xyz
    issuing_account:
      description: The AWS account under which the grant was issued
      type: str