Fixed two Bazel java tests on Windows by using the right native file system

Newly passing:
//src/test/java/com/google/devtools/build/...
  lib/skyframe:SkyframeTests
  lib:actions_test

Also refactored FileSystems.java

--
Change-Id: I03ab9db5c1ab5e5be4ff1efbc5cf2d280084254a
Reviewed-on: https://bazel-review.googlesource.com/#/c/3843
MOS_MIGRATED_REVID=125449456
diff --git a/src/test/java/com/google/devtools/build/lib/vfs/UnixPathTest.java b/src/test/java/com/google/devtools/build/lib/vfs/UnixPathTest.java
index cd174c6..2be5055 100644
--- a/src/test/java/com/google/devtools/build/lib/vfs/UnixPathTest.java
+++ b/src/test/java/com/google/devtools/build/lib/vfs/UnixPathTest.java
@@ -53,7 +53,7 @@
   private File tmpDir;
 
   protected FileSystem getUnixFileSystem() {
-    return FileSystems.initDefaultAsNative();
+    return FileSystems.getNativeFileSystem();
   }
 
   @Before