Added MOZ_ANDROID_SYNTHAPKS build flag. r? nalexander
Initial commit of WebAppDispatcher.

Catches the intent from the synthesized APK, then redirects it to an internal WebApp slot.

The WebApp (actually, the code is in WebAppImpl) displays a splash screen, then sends a message to gecko.

The message is constructed from the contents of the manifest of the synthetic APK, by InstallHelper.