[Mono.Android] Add target to build additional API levels

[pjcollins]

Commit e4a75a8 removed some make magic that ensured Mono.Android.dll
would be built multiple times for additional supported API levels.

A couple of MSBuild tasks used to build multiple versions of
Mono.Android.dll have been moved out of the PackDotNet target used by
the Windows build and into the Mono.Android build to fix this.

[pjcollins]

@jpobst can you try this out in #9594 to see if it can be used as a more permanent fix to the build there?

[jpobst]

@jpobst can you try this out in #9594 to see if it can be used as a more permanent fix to the build there?

Updated my PR with this code (and removed the other workaround).

Results in:

(_BuildAdditionalApiLevels target) -> 
D:\a\_work\1\s\src\Mono.Android\Mono.Android.targets(245,5): error MSB4006: There is a circular dependency in the target dependency graph involving target "_BuildAdditionalApiLevels". [D:\a\_work\1\s\src\Mono.Android\Mono.Android.csproj::TargetFramework=net9.0]

https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=10682236&view=logs&j=cac0e8d3-0ef5-5d2b-b57e-e8fde7204df3&t=be422736-ce1e-556a-8845-3aca534882e6

I think the PR is largely done aside from fixing this issue, so you are welcome to commit fixes to #9594 if you want.

[pjcollins]

Closing this and will continue on 9594.