##### 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
returned: always
      sample: arn:aws:iam::01234567890:root
    key_id:
      description: The key ARN to which the grant applies.
      type: str
      returned: always
      sample: arn:aws:kms:ap-southeast-2:123456789012:key/abcd1234-abcd-1234-5678-ef1234567890
    name:
      description: The friendly name that identifies the grant
      type: str
      returned: always
      sample: xyz
    operations:
      description: The list of operations permitted by the grant
      type: list
      returned: always
      sample:
        - Decrypt
        - RetireGrant
    retiring_principal:
      description: The principal that can retire the grant
      type: str
      returned: always
      sample: arn:aws:sts::0123456789012:assumed-role/lambda_xyz/xyz
changes_needed:
  description: grant types that would be changed/were changed.
  type: dict
  returned: always
  sample: { "role": "add", "role grant": "add" }
had_invalid_entries: