Support specifying a path to an existing AndroidManifest.xml file

Hello @soywiz.
Thanks for your great Game engine. I'm using it for my [Open Source 2048 game](https://github.com/andstatus/game2048/blob/master/README.md#readme) and I recently updated it to the latest KorGe builds...

I could even technically customize AndroidManifest.xml as I need, but current approach with several "injection points" into auto-generated AndroidManifest.xml file is ugly.
Please see how I managed to have my own custom MyMainActivity instead of auto-generated one:
https://github.com/andstatus/game2048/blob/c63b7358cb82c9282695148e0a76a395ec5b057a/build.gradle.kts#L44

Please allow to configure a path to a complete AndroidManifest.xml file, and in a case that custom AndroidManifest.xml file exists:
* use the provided AndroidManifest.xml instead
* skip generation of MainActivity.kt (assuming that provided AndroidManifest.xml takes care of it)

PS: And I do have another reason to have my static AndroidManifest.xml and MainActivity in another folder: to be able to publish my app as a simple "Android app" to F-Droid repository, with its own Gradle build file... (I will need to figure out how to update dependencies there yet...)

Start by tracing the Android build configuration from the referenced build.gradle.kts and locate where AndroidManifest.xml and MainActivity.kt are generated. Define the configuration path for a complete manifest, then verify that the supplied manifest is used and MainActivity.kt generation is skipped when it exists.

Written by the indexing model from the issue text.