The master password isn't actually stored, but it's used to encrypt the master password file, which lives in the security.xml file with your Android Studio preferences. That file contains encrypted versions of your keystore and key passwords if you check the "Remember Passwords" checkbox in the Generate Signed APK wizard; if you don't tell it to remember passwords, nothing is stored.
Where that file lives depends on your platform, but in general on Windows it will be in <User home>\.AndroidStudioPreview\config\security.xml
, ~/Library/Preferences/AndroidStudioPreview/options/security.xml
on MacOS, and ~/.AndroidStudioPreview/config/security.xml
on Linux. See http://www.jetbrains.com/idea/webhelp/project-and-ide-settings.html for more information on where preferences are stored for IntelliJ; it's essentially the same for Android Studio.
You can find a little more information on the file in a SO question about trying to decrypt the file contents if the master password is known: Intellij IDEA - view saved password