Clean up Build targets to remove Legacy Support for Xamarin.

### Android framework version

net9.0-android

### Affected platform version

All

### Description

We should clean up our .targets files to completely remove support for Xamarin. Also take the time to reorganise the Xamarin.Android.Build.Common.targets into seperete targets files.

### Steps to Reproduce

n/a

### Did you find any workaround?

_No response_

### Relevant log output

_No response_