Double post
As mentioned here, you don't want to use PWM to control the ESC. You'll want to use the servo library and control the motors as if they were continuous rotation servos.
Forum member kas wrote "Joystick Bluetooth Commander" app. This app is available for free. Kas has posted code to the forum showing how to use it with an Arduino.
You'll want to use the HC-05 in slave mode with using it with an Arduino.
App Inventor 2 makes it relatively easy to write simple Bluetooth apps for the Android. I attempted my own version of Joystick Bluetooth Commander using AI2 but it didn't turn out as well as kas' app.