Work in Progress. Succeeding in installing the web app, and being returned  back to Android, however, cannot craft an intent and intent filter to launch the app.

The current intent-filter in fennec doesn't actually do anything, given that the GeckoAppShell.getWebAppIntent() names the class and component to launch, i.e. by-passing the intent-filter all together.

The intention was to have a Launcher catch the Launch intent (when the user launches the app), check if we've installed the app, and if so, call out to WebAppInstaller in Fennec. This would then return a result back to LauncherActivity.onActivityResult(). However, android:noHistory prevents the result ever being returned, so we need to implement a new scheme, e.g.
SynthAPK.LauncherActivity -(launches)-> SynthAPK.InstallerActivity -(launches)-> Fennec.WebappInstaller -(returns to)-> SynthAPK.InstallerActivity -(launches)-> SynthAPK.LauncherActivity -(launches)-> Fennec.Webapp$1.

Where LauncherActivity has android:noHistory, but InstallerActivity has history.