As
@DonManfred already told you, your FCM API key should be safely stored on your server. This is the gold rule.
Another approach that you may consider, if you absolutely have to send a message from your mobile app without a mid-layer on a server, is the following:
1. store the FCM key in your on-line DB
2. read at app's startup (or when needed) the key in a temp variable
3. use the key to send your message
This way the key won't plainly show in your code, but anyone disassembling the code will find how to retrieve it from your DB.