Short answer :
Obfuscate or Encrypt your google map API key according to Google Map Documentation :
On mobile apps that use Maps Web Service APIs, consider one or more of the following techniques to further safeguard your apps and API
keys:
Apply an API restriction on the API key. This action narrows the scope of the API key to the APIs you associate with the key.
Obfuscate or encrypt the API key. This action complicates key
scraping attempts directly from the application.
Use CA pinning or certificate pinning to verify the server resources are valid. CA pinning checks that a server's certificate
was issued by a trusted certificate authority, and prevents
Man-In-The-Middle attacks that could lead to a third party
discovering your API key. Certificate pinning goes further by
extracting and checking the public key included in the server
certificate. Pinning is useful for mobile clients communicating
directly with Google servers, as well as mobile clients
communicating with the developer's own proxy server.
Use a proxy server.
You can use many encryption or obfuscation ways you can find easily with just a quick search, One common way is to base64 encoding the key in C++ as a library and to use it just call a function in your Java class, because C++ codes after compiling is more hard to decompile than your Java classes.
Tutorial of doing it with NDK
Long Answer: IT DEPENDS
You can not do anything in client side to proof your code against decompiling and specially make one hard coded string immune to get extracted.
So what you should do ?
It's a trade-off, at First you should see how much your Key is important for you, Who will try to access it and how much time it worth for him to put on hacking your app. With Encryption or obfuscation we just make it harder, So we need more professional person with more time to hack it(Basically with every layer of security we add to it we are doing that.)
If it's too secret and if It's leak will have cause many problem for you and it's critical you should store it in a server, Then you should request to that server and that server will do API call with your key Or with something like a JWT Token.
But in case of Google Map API as long as you monitor it and configure it correctly, leaking your key will not cause many problems for you.