Strip native libraries as part of `Package` target

Context: https://github.com/dotnet/runtime/issues/45013

Currently, the native libraries in the .NET 6 Runtime Pack are not `strip`d, which means:

1. They contain debug symbols, and thus
2. Are bigger.

*Much* bigger:

```
% ls -lh ~/.nuget/packages/microsoft.netcore.app.runtime.android-arm64/6.0.0-alpha.1.20560.10/runtimes/android-arm64/native/libmonosgen-2.0.so
-rwxr--r-- 1 jon staff 17M Nov 11 02:20 /Users/jon/.nuget/packages/microsoft.netcore.app.runtime.android-arm64/6.0.0-alpha.1.20560.10/runtimes/android-arm64/native/libmonosgen-2.0.so

% cp ~/.nuget/packages/microsoft.netcore.app.runtime.android-arm64/6.0.0-alpha.1.20560.10/runtimes/android-arm64/native/libmonosgen-2.0.so libmono-stripped.so
% /Library/Frameworks/Xamarin.Android.framework/Versions/Current//lib/xamarin.android/xbuild/Xamarin/Android/Darwin/ndk/aarch64-linux-android-strip libmono-stripped.so

% ls -lh libmono-stripped.so
-rwxr--r-- 1 jon staff 2.9M Jan 28 11:24 libmono-stripped.so
```

The un-`strip`d `libmonosgen-2.0.so` is 17MB, while the `strip`d version is 2.9MB.

Quite a size difference!

There is a suggestion that the .NET 6 runtime packs should contain `strip`d binaries.

A problem with that is that un`-strip`d binaries are useful! If the app crashes, debug symbols will provide *useful information* about where the crash occurred. If the .NET 6 runtime pack binaries are `strip`d, then *some other mechanism* will need to be provided so that un-`strip`d binaries can be obtained.

*Instead*, let's update the `Package` target to (optionally!) `strip` *all* `.so` files that are added to the `.apk`.

This means that if 3rd party native libraries are shipped as un-`strip`d binaries, they can *also* be handled automagically.

Additionally, the current Xamarin.Android SDK "handles" this problem by shipping both `strip`d `.so` files, and un-`strip`d `.d.so` files:

```
% ls -lh /Library/Frameworks/Xamarin.Android.framework/Versions/Current//lib/xamarin.android/xbuild/Xamarin/Android/lib/arm64-v8a/ | grep monosgen
-rwxr-xr-x 1 root wheel 18M Oct 12 18:11 libmonosgen-2.0.d.so
-rwxr-xr-x 1 root wheel 3.8M Oct 12 18:11 libmonosgen-2.0.so
```

If we uniformly did the `strip` at `Package` time, then we could ship only the debug version, named `libmonosgen-2.0.so`, thus saving ~3.8MB * 4 = ~15MB from our installation size.

(Downside: it means that the `Package` target takes longer, as it'll be doing more work.)