**Apache Airflow version**:

2.0.1

**What happened**:

When clearing task across dags using `ExternalTaskMarker` the dag state of the external `DagRun` is not set to active. So cleared tasks in the external dag will not automatically start if the `DagRun` is a Failed or Succeeded state.

**What you expected to happen**:

The external `DagRun` run should also be set to Running state.

**How to reproduce it**:

Clear tasks in an external dag using an ExternalTaskMarker.


**Anything else we need to know**:

Looking at the code is has:

https://github.com/apache/airflow/blob/b23fc137812f5eabf7834e07e032915e2a504c17/airflow/models/dag.py#L1323-L1335

It seems like it intentionally calls the dag member method `set_dag_run_state` instead of letting the helper function `clear_task_insntances` set the `DagRun` state. But the member method will only change the state of `DagRun`s of dag where the original task is, while I believe `clear_task_instances` would correctly change the state of all involved `DagRun`s.

tests/sensors/test_external_task_sensor.py
"""
    Create a DagBag containing one DAG, with task "head" depending on task "tail" of the
    previous execution_date.
    20200501     20200502                 20200510
    +------+     +------+                 +------+
    | head |    -->head |    -->         -->head |
    |  |   |   / |  |   |   /           / |  |   |
    |  v   |  /  |  v   |  /           /  |  v   |
    | body | /   | body | /     ...   /   | body |
    |  |   |/    |  |   |/           /    |  |   |
    |  v   /     |  v   /           /     |  v   |
    | tail/|     | tail/|          /      | tail |
    +------+     +------+                 +------+
    """
    dag_bag = DagBag(dag_folder=DEV_NULL, include_examples=False)
    with DAG("head_tail", start_date=DEFAULT_DATE, schedule_interval="@daily") as dag:
        head = ExternalTaskSensor(