'Add' support for Android GameActivity entry point

[timbo-tj]

Following the examples the application failed to launch properly in newer version of Unity. This is because the newer versions of Unity use the GameActivity entry point, and the built in AndroidManifest.xml is 'hardcoded' to expect the regular activity.

• I've removed the need to use a custom AndroidManifest.xml with a post-build patching script.
• This script players nicely with other frameworks/plugins. It does not touch the manifest in your project (No version control noise), and instead patches the one that is used for the final build as part of the build pipeline. This is completely transparent to the user.
• I've updated the readme and remove the manifest edit instructions since this is no longer required.
• I've updated the docs to explain what the patcher is doing and how to do it manually if (for whatever reason) you need to do it yourself.

The end result is that there is no AndroidManifest.xml edits required any more. You can build the game with the Activity entry point or the GameActivity entry point, and it'll 'just work'.