[Xamarin.Android.Build.Tasks] Keep ICustomMarshaler.GetInstance (#404)

[pjcollins]

• preserve the static ICustomMarshaler GetInstance (string) method
  of types implementing ICustomMarshaler interface

• it avoids errors like:

    ReadlinkTest
    	ReadLink [FAIL] : System.ApplicationException : Custom marshaler 'FileNameMarshaler' does not implement a static GetInstance method that takes a single string parameter and returns an ICustomMarshaler.
    		  at (wrapper managed-to-native) Mono.Unix.Native.Syscall:readlinkat (int,string,byte[],ulong)
    		  at Mono.Unix.Native.Syscall.readlinkat (System.Int32 dirfd, System.String pathname, System.Byte[] buf) [0x0000a] in <75558c6e0e1f444f8a03d1c694a92a08>:0
    		  at MonoTests.Mono.Unix.ReadlinkTest.SetUp () [0x00007] in <9df8009b87984e1cb863e771d21f9f39>:0
    		  at (wrapper managed-to-native) System.Reflection.MonoMethod:InternalInvoke (System.Reflection.MonoMethod,object,object[],System.Exception&)
    		  at System.Reflection.MonoMethod.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x00038] in <9f55149ed93f4bad9226ce1d7634fe5b>:0
    	readlink_byte [FAIL] : System.ApplicationException : Custom marshaler 'FileNameMarshaler' does not implement a static GetInstance method that takes a single string parameter and returns an ICustomMarshaler.
    TearDown : System.ArgumentException : ** Unknown error code: 9**
      ----> Mono.Unix.UnixIOException : ** Unknown error code: 9** [EBADF].
    		  at (wrapper managed-to-native) Mono.Unix.Native.Syscall:readlinkat (int,string,byte[],ulong)
    		  at Mono.Unix.Native.Syscall.readlinkat (System.Int32 dirfd, System.String pathname, System.Byte[] buf) [0x0000a] in <75558c6e0e1f444f8a03d1c694a92a08>:0
    		  at MonoTests.Mono.Unix.ReadlinkTest.SetUp () [0x00007] in <9df8009b87984e1cb863e771d21f9f39>:0
    		  at (wrapper managed-to-native) System.Reflection.MonoMethod:InternalInvoke (System.Reflection.MonoMethod,object,object[],System.Exception&)
    		  at System.Reflection.MonoMethod.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x00038] in <9f55149ed93f4bad9226ce1d7634fe5b>:0
    		--TearDown
    		  at Mono.Unix.UnixMarshal.ThrowExceptionForLastError () [0x00006] in <75558c6e0e1f444f8a03d1c694a92a08>:0
    		  at MonoTests.Mono.Unix.ReadlinkTest.TearDown () [0x00011] in <9df8009b87984e1cb863e771d21f9f39>:0
    		  at (wrapper managed-to-native) System.Reflection.MonoMethod:InternalInvoke (System.Reflection.MonoMethod,object,object[],System.Exception&)
    		  at System.Reflection.MonoMethod.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x00038] in <9f55149ed93f4bad9226ce1d7634fe5b>:0
    		--UnixIOException
  

[pjcollins]

Cherry pick the fix for Bug 52991 (aka Bug 51340) to d15-1.