Cert: Cleanup and add metadata tag for L2capTest

* Remove unused imports and re-group imports per PEP 8
* Add pts_test_id and pts_test_name metadata tag for L2capTest
* Add "Test Purpose" as doc strings of test
* In general, a PTS test should look like:

@metadata(pts_test_id="{PTS_TEST_ID}",
          pts_test_name="{PTS_TEST_NAME}")
def test_name_in_acts(self):
    """
    {PTS_TEST_DESCRIPTION}
    """
    # Test logic

Test: source gd/cert/run --host --test_filter=L2capTest
Bug: 152828141
Change-Id: Ia3da883f6100b9d3937fcfb6ca419b38869a623f