I recently updated to Android Studio 2.3, and now when I try to run the application, the emulator does not come online. It times out after 300 seconds.
Additionally,
I had this issue twice and tried everything in this thread with no results, I finally fixed it closing "BlueStacks" emulator.
If BlueStacks (and maybe others android emulators) are running when I run the app, by default Android Studio installs the app there, even when I never selected it as a device in AVD Manager, then Android Studio waits for an incorrect device "to come online".
Closing BlueStacks (or any other Android emulator) solved the conflict between devices for me.
I hope this info helps someone!