##### 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
existing_dict = dict((eg['name'], eg) for eg in existing_grants)
    desired_dict = dict((dg['name'], dg) for dg in desired_grants)
    to_add_keys = set(desired_dict.keys()) - set(existing_dict.keys())
    if purge_grants:
        to_remove_keys = set(existing_dict.keys()) - set(desired_dict.keys())
    else:
        to_remove_keys = set()
    to_change_candidates = set(existing_dict.keys()) & set(desired_dict.keys())
    for candidate in to_change_candidates:
        if different_grant(existing_dict[candidate], desired_dict[candidate]):
            to_add_keys.add(candidate)
            to_remove_keys.add(candidate)
    to_add = []
    to_remove = []
    for key in to_add_keys:
        grant = desired_dict[key]
        to_add.append(grant)
    for key in to_remove_keys:
        grant = existing_dict[key]
        to_remove.append(grant)
    return to_add, to_remove
def ensure_enabled_disabled(connection, module, key):