Rename `get_e2e_device_keys` to better reflect its purpose (#8205)

... and to show that it does something slightly different to
`_get_e2e_device_keys_txn`.

`include_all_devices` and `include_deleted_devices` were never used (and
`include_deleted_devices` was broken, since that would cause `None`s in the
result which were not handled in the loop below.

Add some typing too.