##### 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
# Update previous key with more tags
- aws_kms:
    alias: mykey
    tags:
      Name: myKey
      Purpose: protect_stuff
      Owner: security_team
# Update a known key with grants allowing an instance with the billing-prod IAM profile
# to decrypt data encrypted with the environment: production, application: billing
# encryption context
- aws_kms:
    key_id: abcd1234-abcd-1234-5678-ef1234567890
    grants:
      - name: billing_prod
        grantee_principal: arn:aws:iam::1234567890123:role/billing_prod
        constraints:
          encryption_context_equals:
            environment: production
            application: billing
        operations:
          - Decrypt
          - RetireGrant
'''
RETURN = '''
key_id:
  description: ID of key
  type: str
  returned: always
  sample: abcd1234-abcd-1234-5678-ef1234567890
key_arn: