Fix notifications not working in Android emulator

Fixes #3113

The native code inside `react-native-notifications` varied its
behavior when reading the `gcm_id` value from the metadata. Because
of this it was throwing a `MismatchSenderId` error.

One solution that was suggested in the GitHub's Issues was renaming
`android:value="your_sender_id\0"` to `android:value="your_sender_id\"`
This worked but I was a bit unconmfortable with the contrary suggestion
in the official documentation `// IMPORTANT: Leave the trailing \0 intact!!!`

Instead, extract this value to a string resource and refer it by name.

android/app/src/main/AndroidManifest.xml

@@ -67,7 +67,6 @@
                 <category android:name="com.zulipmobile" />
             </intent-filter>
         </receiver>
-        <meta-data android:name="com.wix.reactnativenotifications.gcmSenderId" android:value="835904834568\0"/>
+        <meta-data android:name="com.wix.reactnativenotifications.gcmSenderId" android:value="@string/gcm_id"/>
     </application>
-
 </manifest>

android/app/src/main/res/values/strings.xml

@@ -1,6 +1,7 @@
 <resources>
     <string name="app_name">Zulip</string>
     <string name="notification_channel_name">Notifications</string>
+    <string name="gcm_id">835904834568</string>
     <string name="no_browser_found">No Browser Found</string>
     <plurals name="messages">
         <item quantity="one"></item>