[android] don't probe directories for .config files (#18024)

[android] don't probe directories for .config files (#18024)

* [android] don't probe directories for .config files

In Xamarin.Android, on startup we see logging like this for every .NET
assembly:

    12-03 14:27:42.104 26842 26842 D Mono    : Prepared to set up assembly 'FormsViewGroup' (/storage/emulated/0/Android/data/Xamarin.Forms_Performance_Integration/files/.__override__/FormsViewGroup.dll)
    12-03 14:27:42.104 26842 26842 D Mono    : Assembly FormsViewGroup[0xebcee9c0] added to domain RootDomain, ref_count=1
    12-03 14:27:42.104 26842 26842 D Mono    : Assembly Loader loaded assembly from location: '/storage/emulated/0/Android/data/Xamarin.Forms_Performance_Integration/files/.__override__/FormsViewGroup.dll'.
    12-03 14:27:42.104 26842 26842 D Mono    : Config attempting to parse: '/storage/emulated/0/Android/data/Xamarin.Forms_Performance_Integration/files/.__override__/FormsViewGroup.dll.config'.
    12-03 14:27:42.104 26842 26842 D Mono    : Config attempting to parse: '/Users/builder/jenkins/workspace/archive-mono/2019-10/android/release/sdks/out/android-x86-release/etc/mono/assemblies/FormsViewGroup/FormsViewGroup.config'.

In most cases, no `.dll.config` file exists and so it probes this
macOS-looking path:

    /Users/builder/jenkins/workspace/archive-mono/2019-10/android/release/sdks/out/android-x86-release/etc/mono/assemblies/FormsViewGroup/FormsViewGroup.config

It looks like this is happening because nothing in Xamarin.Android
calls `mono_set_dirs` or `mono_set_config_dir` and so it falls back to
using `MONO_CFG_DIR`. `MONO_CFG_DIR` is set to a path on the build
machine.

Even if Xamarin.Android *did* call `mono_set_config_dir`, the code
appends `mono/assemblies`:

    cfg = g_build_filename (cfg_dir, "mono", "assemblies", aname, cfg_name, (const char*)NULL);

So it doesn't seem useful to use this fallback at all, there would
never be a file found.

I wrapped the appropriate code in `#ifndef TARGET_ANDROID` blocks.

I was able to see a minor *maybe* ~20ms performance improvement on
startup:

    Before:
    12-03 14:27:48.494 26903 26903 I monodroid-timing: Runtime.init: end, total time; elapsed: 0s:601::443900
    12-03 14:27:54.356 26963 26963 I monodroid-timing: Runtime.init: end, total time; elapsed: 0s:587::495400
    12-03 14:28:00.263 27021 27021 I monodroid-timing: Runtime.init: end, total time; elapsed: 0s:586::147100
    12-03 14:28:06.135 27074 27074 I monodroid-timing: Runtime.init: end, total time; elapsed: 0s:593::639100
    12-03 14:28:11.988 27129 27129 I monodroid-timing: Runtime.init: end, total time; elapsed: 0s:571::194900
    After:
    12-03 14:46:50.590 27737 27737 I monodroid-timing: Runtime.init: end, total time; elapsed: 0s:503::918700
    12-03 14:46:56.517 27794 27794 I monodroid-timing: Runtime.init: end, total time; elapsed: 0s:570::533600
    12-03 14:47:02.368 27850 27850 I monodroid-timing: Runtime.init: end, total time; elapsed: 0s:537::435000
    12-03 14:47:08.289 27904 27904 I monodroid-timing: Runtime.init: end, total time; elapsed: 0s:587::830900
    12-03 14:47:14.130 27965 27965 I monodroid-timing: Runtime.init: end, total time; elapsed: 0s:555::593400

This was the Xamarin.Forms integration project in xamarin-android.
Running a `Debug` build on a HAXM x86 emulator on Windows.

https://github.com/xamarin/xamarin-android/tree/master/tests/Xamarin.Forms-Performance-Integration

* Add --enable-minimal=cfgdir_config

* This also defines `DISABLE_CFGDIR_CONFIG`
* Used this setting for android