RMF: PrivacyShield placeholder (#4620)

Task/Issue URL: https://app.asana.com/0/1201807753394693/1207502949571218/f

### Description
Added new placeholder image for RMF

### Steps to test this PR

_Pre steps_
- [ ] Change URL in `RemoteMessagingService` for `http://www.jsonblob.com/api/1248254773076811776`

_PrivacyShield placeholder_
- [ ] Fresh install from branch
- [ ] Go to a website
- [ ] Dismiss trackers dialogs
- [ ] Open a new tab
- [ ] Perform a search
- [ ] Open a new tab
- [ ] Check new PrivacyShield placeholder image is correct

### UI changes
| Light  | Dark |
| ------ | ----- |
![Screenshot_20240606_134124_DuckDuckGo](https://github.com/duckduckgo/Android/assets/20798495/36065d6b-2b28-4de7-a328-4d42ec23e0b1)|![Screenshot_20240606_134054_DuckDuckGo](https://github.com/duckduckgo/Android/assets/20798495/166f929d-538c-4216-9931-421b0573f6cd)|

app/src/main/java/com/duckduckgo/app/browser/remotemessage/RemoteMessageMapper.kt

@@ -28,6 +28,7 @@ import com.duckduckgo.remote.messaging.api.Content.Placeholder.APP_UPDATE
 import com.duckduckgo.remote.messaging.api.Content.Placeholder.CRITICAL_UPDATE
 import com.duckduckgo.remote.messaging.api.Content.Placeholder.DDG_ANNOUNCE
 import com.duckduckgo.remote.messaging.api.Content.Placeholder.MAC_AND_WINDOWS
+import com.duckduckgo.remote.messaging.api.Content.Placeholder.PRIVACY_SHIELD
 import com.duckduckgo.remote.messaging.api.Content.PromoSingleAction
 import com.duckduckgo.remote.messaging.api.Content.Small
 import com.duckduckgo.remote.messaging.api.RemoteMessage
@@ -77,5 +78,6 @@ private fun Placeholder.drawable(): Int {
         CRITICAL_UPDATE -> R.drawable.ic_critical_update
         APP_UPDATE -> R.drawable.ic_app_update
         MAC_AND_WINDOWS -> R.drawable.desktop_promo_artwork
+        PRIVACY_SHIELD -> R.drawable.ic_privacy_pro
     }
 }

remote-messaging/remote-messaging-api/src/main/java/com/duckduckgo/remote/messaging/api/RemoteMessage.kt

@@ -81,6 +81,7 @@ sealed class Content(val messageType: MessageType) {
         CRITICAL_UPDATE("CriticalUpdate"),
         APP_UPDATE("AppUpdate"),
         MAC_AND_WINDOWS("NewForMacAndWindows"),
+        PRIVACY_SHIELD("PrivacyShield"),
         ;
 
         companion object {