amIWebInstallPrompt is used, if available, by the default implementation of
amIWebInstallInfo to display a confirmation UI to the user before running
installs.
Get a confirmation that the user wants to start the installs.
| aWindow | The window that triggered the installs |
| aUri | The URI of the site that triggered the installs |
| aInstalls | The AddonInstalls that were requested |
| aCount | The number of AddonInstalls |