Firstly, Add this android:debuggable="false" in the application tag of the AndroidManifest.xml.
You don't need to harcode android:debuggable="false" in your application tag. Infact for me studio complaints -
Avoid hardcoding the debug mode; leaving it out allows debug and release builds to automatically assign one less... (Ctrl+F1)
It's best to leave out the android:debuggable attribute from the manifest. If you do, then the tools will automatically insert android:debuggable=true when building an APK to debug on an emulator or device. And when you perform a release build, such as Exporting APK, it will automatically set it to false. If on the other hand you specify a specific value in the manifest file, then the tools will always use it. This can lead to accidentally publishing your app with debug information.
The accepted answer looks somewhat old. For me it asks me to select whether I want debug build or release build.
Go to Build->Generate Signed APK. Select your keystore, provide keystore password etc.
Now you should see a prompt to select release build or debug build.
For production always select release build!
And you are done. Signed APK exported.
PS : Don't forget to increment your versionCode in manifest file before uploading to playstore :)