Bug 1869558 - Part 1: Add validation step for subscribeWithKey r=jonalmeida,geckoview-reviewers

Ideally this validation steps should be shared in PushManager::Subscribe, but that's out of scope for this bug. For now this just copypastes the step from `dom/push/PushService.sys.mjs`.

Differential Revision: https://phabricator.services.mozilla.com/D196586

mobile/android/components/geckoview/GeckoViewPush.sys.mjs

@@ -10,6 +10,7 @@ const lazy = {};
 
 ChromeUtils.defineESModuleGetters(lazy, {
   EventDispatcher: "resource://gre/modules/Messaging.sys.mjs",
+  PushCrypto: "resource://gre/modules/PushCrypto.sys.mjs",
 });
 
 // Observer notification topics for push messages and subscription status
@@ -67,13 +68,22 @@ export class PushService {
   }
 
   async subscribeWithKey(scope, principal, appServerKey, callback) {
+    const keyView = new Uint8Array(appServerKey);
+
+    try {
+      await lazy.PushCrypto.validateAppServerKey(keyView);
+    } catch (error) {
+      callback.onPushSubscription(Cr.NS_ERROR_DOM_PUSH_INVALID_KEY_ERR, null);
+      return;
+    }
+
     try {
       const response = await lazy.EventDispatcher.instance.sendRequestForResult(
         {
           type: "GeckoView:PushSubscribe",
           scope: scopeWithAttrs(scope, principal.originAttributes),
           appServerKey: appServerKey
-            ? ChromeUtils.base64URLEncode(new Uint8Array(appServerKey), {
+            ? ChromeUtils.base64URLEncode(keyView, {
                 pad: true,
               })
             : null,