_AppBundlePath is relative by default, but if AppBundleDir is set to an
absolute path (or the root path used to compute AppBundleDir is an absolute
path), then we must ensure _AppBundlePath is still a relative path, because
our code depends on this.
Note that there are no tests for this, because:
* The problem is that our code concatenates a path + _AppBundlePath.
* This works fine on macOS, because it still looks like a valid path.
* It does not work fine on Windows, because the resulting path ends up with a
drive letter in the middle.
* We currently don't have any tests on Windows.
Fixes https://github.com/xamarin/xamarin-macios/issues/15130.
f31f53f849