Key storage out of sync: reset key backup when needed (#31279)

* add function to pause device listener

* add function to check if key backup key missing both locally and in 4s

* reset backup if backup key missing both locally and in 4s

* fixup! add function to check if key backup key missing both locally and in 4s

* Drop KEY_STORAGE_OUT_OF_SYNC_STORE in favour of checking cross-signing

Check if cross-signing needs resetting, because that seems to be what
KEY_STORAGE_OUT_OF_SYNC_STORE is actually trying to do.

* add a function for resetting key backup and waiting until it's ready

* trigger key storage out of sync toast when missing backup key locally

and fetch it when user enters their recovery key

* reset backup when needed if user forgets recovery key

* rename function as suggested in code review

test/test-utils/test-utils.ts

@@ -195,6 +195,7 @@ export function createTestClient(): MatrixClient {
                 content: {},
             });
         }),
+        getAccountDataFromServer: jest.fn(),
         mxcUrlToHttp: jest.fn().mockImplementation((mxc: string) => `http://this.is.a.url/${mxc.substring(6)}`),
         setAccountData: jest.fn(),
         deleteAccountData: jest.fn(),