After loading the model onto the phone from Simulink, the Simulink model is already installed as an app on the phone. The app's APK file has the same name as the Simulink model. You may locate it by browsing apps on the phone.
The default APK directory for recent versions of Android is /data/app/directory. You will have to be rooted on the device to view/access those locations.
When you deploy the model to the device, Simulink is creating the APK file as part of the code generation process. To generate code without loading the model to the phone, select the following option from Model Configuration parameters : Hardware Implementation -> Build options -> 'Build'. This is opposed to the default 'Build, Load, and Run' option.
After that, generate code (ctrl+B) and the APK file will be created in the folder:
If you want to install the app without using Simulink, or install it to any other phone, you can copy this APK from the host computer directly to the Android file system, then install and use it like any other app. You may have to change one of your phone's settings to allow 3rd party APKs to be installed.
The following article may be of some assistance regarding installing 3rd party apps for Android: