You can ignore the limitation, but we can’t guarantee that it will work properly and do not officially support or test the SDK in this configuration ourselves.
If you want to build a fully-custom UI for your messaging interface, you can easily create one using Sunshine Conversations to manage message sending and receiving.
You’ll want to look at Conversation.java & Conversation delegate detailed in the Android SDK Reference.
To do so, you’ll need to create your own activity that extends ConversationActivity.
Once you’ve created your own activity, ensure that it’s launched when a push notification is received.