Strip AOT shared libraries when linking

[grendello]

Fixes: #6840
Context: #6683

Pass the -s flag to the native linker in order to produce shared
AOT libraries without debug symbols. Symbols are stripped unless
the $(DebugSymbols) property is set to True.

[jonathanpeppers]

This seems pretty good!

Size difference in bytes ([*1] apk1 only, [*2] apk2 only):

 17,416 lib/armeabi-v7a/libaot-Mono.Security.dll.so *2
 11,356 lib/x86/libaot-Mono.Security.dll.so *2
Error: apkdiff: PackageSize decrease 3,413,474 is 3,364,322 bytes more than the threshold 49,152. apk1 size: 19,475,110 bytes, apk2 size: 16,061,636 bytes.

And this was a classic Xamarin.Andorid/Xamarin.Forms apps.

[jonpryor]

Fixes: https://github.com/xamarin/xamarin-android/issues/6840

Context: b21cbf943a19079773f9e762908939181a7f2180

Commit b21cbf94 contained a TODO:

> TODO: the Resulting `.apk` sizes also increase[d] unexpectedly, with
> `Xamarin.Forms_Performance_Integration-Signed-Release-Profiled-Aot.apkdesc`
> showing a 1.7MB increase in `.apk` size.  We believe that this is
> because of more verbose debug symbols.
> xamarin/xamarin-android#6840 will track this.

Update the `<GetAotArguments/>` MSBuild task to add the `-s` linker
flag to `GetAotArguments.LdFlags`.  This will cause the native linker
to produce shared AOT libraries without debug symbols.

Debug symbols are stripped unless the `$(DebugSymbols)`=True.

This fixes the size regression in
`Xamarin.Forms_Performance_Integration-Signed-Release-Profiled-Aot.apkdesc`,
shrinking PackageSize from 19,475,110 down to 16,061,636.
Compare to the pre-b21cbf94 PackageSize of 17,713,830: we're now
1.6MB *smaller* than b21cbf94!