"make framework-assemblies" should not fail by broken framework lookup

[atsushieno]

make framework-assemblies is to get all the frameworks and mono runtimes for each ABI and it need to run successfully (at least to build product). However it has been broken for months and it's never fixed:

"/sources/xamarin-android/src/Mono.Android/Mono.Android.csproj" (default target) (1) ->
(GetReferenceAssemblyPaths target) -> 
  /devel/lib/mono/msbuild/15.0/bin/Microsoft.Common.CurrentVersion.targets(1140,5): error MSB3644:
 The reference assemblies for framework "MonoAndroid,Version=v1.0" were not found. To resolve 
this, install the SDK or Targeting Pack for this framework version or retarget your application to a 
version of the framework for which you have the SDK or Targeting Pack installed. Note that 
assemblies will be resolved from the Global Assembly Cache (GAC) and will be used in place of 
reference assemblies. Therefore your assembly may not be correctly targeted for the framework 
you intend. [/sources/xamarin-android/src/Mono.Android/Mono.Android.csproj]

make framework-assemblies internally runs make and msbuild many times, and the verbose build log for the most relevant msbuild execution can be found at: https://gist.github.com/atsushieno/1b8861f733c2562953b47f7f59e3527c

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