Fix build with newer cecil (from overriden source path) (dotnet#428)

When trying to use newer mono in XA, we run into issues where cecil
(from mono repo) doesn't build anymore. Context:
dotnet/android#3159

Turned out we need to restore nuget packages for cecil before the
build to avoid errors like:

    Mono.Cecil/TypeSystem.cs(83,29): error CS0518: Predefined type 'System.Object' is not defined or imported [/Users/rodo/git/xa2/external/mono/external/cecil/Mono.Cecil.csproj]
    Mono.Cecil/TypeSystem.cs(83,4): error CS0518: Predefined type 'System.Void' is not defined or imported [/Users/rodo/git/xa2/external/mono/external/cecil/Mono.Cecil.csproj]
    Mono.Cecil/TypeSystem.cs(81,4): error CS0518: Predefined type 'System.Object' is not defined or imported [/Users/rodo/git/xa2/external/mono/external/cecil/Mono.Cecil.csproj]

      8 Warning(s)
      7700 Error(s)

Also grant access from `Mono.Cecil.[Mdb|Pdb]` assemblies to
`Mono.Cecil`. Looks like the access restrictions got tighter and so
CSC was complaining about the wrong key. It happens because the
original cecil key differ from our `product.snk` key, which we add in
our overrides and so access for `Mono.Cecil.[Mdb|Pdb]` granted by
original cecil `AssemblyInfo.cs` file is not valid anymore.

The error we were getting from CSC:

    CSC : error CS0281: Friend access was granted by 'Mono.Cecil, Version=0.11.0.0, Culture=neutral, PublicKeyToken=84e04ff9cfb79065', but the public key of the output assembly does not match that specified by the attribute in the granting assembly. [/Users/rodo/git/xa2/external/mono/external/cecil/symbols/mdb/Mono.Cecil.Mdb.csproj]

external/Mono.Cecil.AssemblyInfo.cs

@@ -2,3 +2,5 @@
 
 [assembly: InternalsVisibleTo ("Xamarin.Android.Cecil.Pdb, PublicKey=0024000004800000940000000602000000240000525341310004000011000000438ac2a5acfbf16cbd2b2b47a62762f273df9cb2795ceccdf77d10bf508e69e7a362ea7a45455bbf3ac955e1f2e2814f144e5d817efc4c6502cc012df310783348304e3ae38573c6d658c234025821fda87a0be8a0d504df564e2c93b2b878925f42503e9d54dfef9f9586d9e6f38a305769587b1de01f6c0410328b2c9733db")]
 [assembly: InternalsVisibleTo ("Xamarin.Android.Cecil.Mdb, PublicKey=0024000004800000940000000602000000240000525341310004000011000000438ac2a5acfbf16cbd2b2b47a62762f273df9cb2795ceccdf77d10bf508e69e7a362ea7a45455bbf3ac955e1f2e2814f144e5d817efc4c6502cc012df310783348304e3ae38573c6d658c234025821fda87a0be8a0d504df564e2c93b2b878925f42503e9d54dfef9f9586d9e6f38a305769587b1de01f6c0410328b2c9733db")]
+[assembly: InternalsVisibleTo ("Mono.Cecil.Pdb, PublicKey=0024000004800000940000000602000000240000525341310004000011000000438ac2a5acfbf16cbd2b2b47a62762f273df9cb2795ceccdf77d10bf508e69e7a362ea7a45455bbf3ac955e1f2e2814f144e5d817efc4c6502cc012df310783348304e3ae38573c6d658c234025821fda87a0be8a0d504df564e2c93b2b878925f42503e9d54dfef9f9586d9e6f38a305769587b1de01f6c0410328b2c9733db")]
+[assembly: InternalsVisibleTo ("Mono.Cecil.Mdb, PublicKey=0024000004800000940000000602000000240000525341310004000011000000438ac2a5acfbf16cbd2b2b47a62762f273df9cb2795ceccdf77d10bf508e69e7a362ea7a45455bbf3ac955e1f2e2814f144e5d817efc4c6502cc012df310783348304e3ae38573c6d658c234025821fda87a0be8a0d504df564e2c93b2b878925f42503e9d54dfef9f9586d9e6f38a305769587b1de01f6c0410328b2c9733db")]

src/Xamarin.Android.Cecil/Xamarin.Android.Cecil.targets

@@ -17,7 +17,7 @@
       Outputs="$(CecilAssemblies)">
     <MSBuild
         Projects="@(_CecilProject)"
-        Targets="Clean;Build"
+        Targets="Clean;Restore;Build"
         StopOnFirstFailure="True"
         Properties="Configuration=net_4_0_Debug;OutputPath=$(CecilOutputPath);BuildingSolutionFile=false" />
     <Touch Files="$(CecilAssemblies)" />