Use `device_one_time_keys_count` to match MSC3202 (#14565)

* Use `device_one_time_keys_count` to match MSC3202

Rename the `device_one_time_key_counts` key in responses to
`device_one_time_keys_count` to match the name specified by MSC3202.

Also change related variable/class names for consistency.

Signed-off-by: Andrew Ferrazzutti <andrewf@element.io>

* Update changelog.d/14565.misc

* Revert name change for `one_time_key_counts` key

as this is a different key altogether from `device_one_time_keys_count`,
which is used for `/sync` instead of appservice transactions.

Signed-off-by: Andrew Ferrazzutti <andrewf@element.io>