Hide default constructor for WorkManager.

Test: Ran tests.
Change-Id: I1ebea0f12a6709b1d3bde6269445fdc430611302
diff --git a/work/workmanager/src/main/java/androidx/work/WorkManager.java b/work/workmanager/src/main/java/androidx/work/WorkManager.java
index f3ddf8e..71a61eb 100644
--- a/work/workmanager/src/main/java/androidx/work/WorkManager.java
+++ b/work/workmanager/src/main/java/androidx/work/WorkManager.java
@@ -19,6 +19,7 @@
 import android.arch.lifecycle.LiveData;
 import android.content.Context;
 import android.support.annotation.NonNull;
+import android.support.annotation.RestrictTo;
 
 import androidx.work.impl.WorkManagerImpl;
 
@@ -207,4 +208,11 @@
      * @return A {@link SynchronousWorkManager} object, which gives access to synchronous methods
      */
     public abstract SynchronousWorkManager synchronous();
+
+    /**
+     * @hide
+     */
+    @RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
+    protected WorkManager() {
+    }
 }