fix(matrix): pass required args to MemoryCryptoStore for mautrix ≥0.21

MemoryCryptoStore.__init__() now requires account_id and pickle_key
positional arguments as of mautrix 0.21. The migration from matrix-nio
(commit 1850747) didn't account for this, causing E2EE initialization
to fail with:

  MemoryCryptoStore.__init__() missing 2 required positional arguments:
  'account_id' and 'pickle_key'

Pass self._user_id as account_id and derive pickle_key from the same
user_id:device_id pair already used for the on-disk HMAC signature.

Update the test stub to accept the new parameters.

Fixes #7803

tests/gateway/test_matrix.py

@@ -157,7 +157,9 @@ def _make_fake_mautrix():
     mautrix_crypto_store = types.ModuleType("mautrix.crypto.store")
 
     class MemoryCryptoStore:
-        pass
+        def __init__(self, account_id="", pickle_key=""):
+            self.account_id = account_id
+            self.pickle_key = pickle_key
 
     mautrix_crypto_store.MemoryCryptoStore = MemoryCryptoStore