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've had the same problem (AVD not coming online) in a Linux system. In my case, I have solved it setting this environment variable:
$ export ANDROID_EMULATOR_USE_SYSTEM_LIBS = 1
This case is documented here: https://developer.android.com/studio/command-line/variables.html#studio_jdk