title: GrantPermissionRule leads to PermissionException
body: <p>I've tried today the new android testing support lib. I need the write external storage permission for my tests so I've created the following test with the new GrantPermissionRule.  </p>
  
  <p>ExternalDirTest.java</p>
  
  <pre><code>    @Rule
      public GrantPermissionRule mRuntimePermissionRule = GrantPermissionRule.grant(Manifest.permission.WRITE_EXTERNAL_STORAGE);
  
      @Test
      public void test() throws IOException {
          Application.getApplication().createDir();
      }
  </code></pre>
  
  <p>Application.java</p>
  
  <pre><code>    public void createDir() throws IOException {
          new File(Environment.getExternalStorageDirectory() + "/test").createNewFile();
      }
  </code></pre>
  
  <p>Officially I get the permission (visible in the app settings). Nevertheless I get the following exception:</p>
  
  <pre><code>java.io.IOException: Permission denied
  at java.io.UnixFileSystem.createFileExclusively0(Native Method)
  at java.io.UnixFileSystem.createFileExclusively(UnixFileSystem.java:280)
  at java.io.File.createNewFile(File.java:948)
  at test.app.directory.Application.createDir(EarApplication.java:253)
  at test.app.directory.ExternalDirTest.test(ExternalDirTest.java:24)
  at java.lang.reflect.Method.invoke(Native Method)
  at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
  at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
  at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
  at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
  at android.support.test.rule.GrantPermissionRule$RequestPermissionStatement.evaluate(GrantPermissionRule.java:109)
  at org.junit.rules.RunRules.evaluate(RunRules.java:20)
  at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
  at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
  at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
  at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
  at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
  at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
  at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
  at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
  at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
  at org.junit.runners.Suite.runChild(Suite.java:128)
  at org.junit.runners.Suite.runChild(Suite.java:27)
  at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
  at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
  at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
  at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
  at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
  at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
  at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
  at org.junit.runner.JUnitCore.run(JUnitCore.java:115)
  at android.support.test.internal.runner.TestExecutor.execute(TestExecutor.java:58)
  at android.support.test.runner.AndroidJUnitRunner.onStart(AndroidJUnitRunner.java:369)
  at android.app.Instrumentation$InstrumentationThread.run(Instrumentation.java:1932)
  </code></pre>
  
  <p>If I test the code manual it works perfectly.</p>
  
  <p>Has someone facing the same problem or is it maybe a bug in the library itself?</p>