Reset ShadowAccessibilityManager's states on teardown

Once SAM is enabled in one test case, all the next test cases runs with
SAM enabled because it affects a static value.

This can be prevented by adding a Resetter.

(Based on brettchabot@'s suggestion at
https://github.com/robolectric/robolectric/issues/3364)

BUG=756707
diff --git a/robolectric-shadows/shadows-core/src/main/java/org/robolectric/shadows/ShadowAccessibilityManager.java b/robolectric-shadows/shadows-core/src/main/java/org/robolectric/shadows/ShadowAccessibilityManager.java
index 287a986..755d657 100644
--- a/robolectric-shadows/shadows-core/src/main/java/org/robolectric/shadows/ShadowAccessibilityManager.java
+++ b/robolectric-shadows/shadows-core/src/main/java/org/robolectric/shadows/ShadowAccessibilityManager.java
@@ -13,6 +13,7 @@
 import org.robolectric.annotation.Implementation;
 import org.robolectric.annotation.Implements;
 import org.robolectric.annotation.HiddenApi;
+import org.robolectric.annotation.Resetter;
 import org.robolectric.util.ReflectionHelpers;
 import org.robolectric.internal.Shadow;
 import org.robolectric.util.ReflectionHelpers.ClassParameter;
@@ -36,6 +37,13 @@
   private List<ServiceInfo> accessibilityServiceList;
   private boolean touchExplorationEnabled;
 
+  @Resetter
+  public static void reset() {
+      synchronized (sInstanceSync) {
+          sInstance = null;
+      }
+  }
+
   @HiddenApi
   @Implementation
   public static AccessibilityManager getInstance(Context context) throws Exception {