[build] fix BuildJniEnvironment_g_cs so it builds incrementally

[jonathanpeppers]

When building xamarin-android, I noticed this always runs when there
are no changes:

Target Name=BuildJniEnvironment_g_cs Project=Java.Interop.csproj
Building target "BuildJniEnvironment_g_cs" completely.
Input file "C:\src\xamarin-android\external\Java.Interop\bin\BuildDebug\jnienv-gen.exe" is newer than output file "Java.Interop/JniEnvironment.g.cs".
...
Exec Task (345ms)

Reviewing the code in jnienv-gen.exe, it might not actually update
either of the Outputs if there are no changes.

So we can either:

1. Add a <Touch/> call in the BuildJniEnvironment_g_cs MSBuild
   target.
2. Make jnienv-gen.exe always update the files.

I think we should just go with option 2, as it was basically doing:

if (!File.Exists (jnienv_g_cs) || !string.Equals (content, File.ReadAllText (jnienv_g_cs)))
    File.WriteAllText (jnienv_g_cs, content);

The string.Equals call doesn't seem terrible efficient anyway--let's
just remove it and always write.

Now we get this on builds without changes:

Skipping target "BuildJniEnvironment_g_cs" because all output files are up-to-date with respect to the input files.

Fixing this seems to save the ~350ms per $(TargetFramework) in
Java.Interop.csproj.

[jpobst]

I guess this part of the BuildJniEnvironment_g_cs target is no longer needed since we now have globs: https://github.com/xamarin/java.interop/blob/master/src/Java.Interop/Directory.Build.targets#L25-L27.

Prior to globs I assume this target needed to run every time in order to add the file to the compilations?

[jonathanpeppers]

@jpobst you could try removing this line:

https://github.com/xamarin/java.interop/blob/25458a5e579154c3a72e16325193010e3ade614f/src/Java.Interop/Directory.Build.targets#L25-L27

But since the file doesn't exist at MSBuild evaluation time and is created by this target, I think it would still be missing from @(Compile).