Receive private key for caching from the app layer

This passes through the new secret storage key as well as the key info so that
bootstrap can use it immediately without triggering an immediate prompt.

Part of element-hq/element-web#12867

src/crypto/index.js

@@ -637,12 +637,13 @@ Crypto.prototype.bootstrapSecretStorage = async function({
             } else {
                 if (!newKeyId) {
                     logger.log("Secret storage default key not found, creating new key");
-                    const keyOptions = await createSecretStorageKey();
+                    const [keyOptions, , key] = await createSecretStorageKey();
                     newKeyId = await this.addSecretStorageKey(
                         SECRET_STORAGE_ALGORITHM_V1_AES,
                         keyOptions,
                     );
                     await this.setDefaultSecretStorageKeyId(newKeyId);
+                    ssssKeys[newKeyId] = key;
                 }
                 if (await this.isSecretStored("m.megolm_backup.v1")) {
                     // we created a new SSSS, and we previously encrypted the