Revert "InputConnectionWrapper never supports null target."

This reverts commit 90bd36363c.

Seems that the semantics of InputConnectionWrapper#setTarget() is more
complicated than I thought.  At least the following cases have worked
fine.

  case 1:
    InputConnectionWrapper wrapper =
            new InputConnectionWrapper(null, false);
    wrapper.SetTarget(ic);
    ...

  case 2:
    InputConnectionWrapper wrapper =
            new InputConnectionWrapper(null, true);
    wrapper.SetTarget(ic);
    ...

  case 3:
    InputConnectionWrapper wrapper =
            new InputConnectionWrapper(ic, true);
    wrapper.SetTarget(null);
    wrapper.SetTarget(ic2);
    ...

The previous code did not intended to break existing code.  Let's revert
it we decide how to deal with above cases.

Bug: 27407697
Change-Id: I8bc84d484ab0b27a02e74f11110430f70646e69a

core/java/android/view/inputmethod/InputConnectionWrapper.java

@@ -16,47 +16,33 @@
 
 package android.view.inputmethod;
 
-import android.annotation.NonNull;
 import android.os.Bundle;
 import android.os.Handler;
 import android.view.KeyEvent;
 
-import static com.android.internal.util.Preconditions.checkNotNull;
-
 /**
  * <p>Wrapper class for proxying calls to another InputConnection.  Subclass
  * and have fun!
  */
 public class InputConnectionWrapper implements InputConnection {
-    @NonNull
     private InputConnection mTarget;
     final boolean mMutable;
-
-    /**
-     * Initializes the wrapper for the given {@link InputConnection}.
-     * @param target the {@link InputConnection} to be wrapped.
-     * @param mutable {@code true} if the wrapper is to be mutable.
-     * @throws NullPointerException if {@code target} is {@code null}.
-     */
-    public InputConnectionWrapper(@NonNull InputConnection target, boolean mutable) {
-        checkNotNull(target);
+    
+    public InputConnectionWrapper(InputConnection target, boolean mutable) {
         mMutable = mutable;
         mTarget = target;
     }
 
     /**
      * Change the target of the input connection.
-     * @param target the {@link InputConnection} to be wrapped.
-     * @throws NullPointerException if {@code target} is {@code null}.
      */
-    public void setTarget(@NonNull InputConnection target) {
-        checkNotNull(target);
+    public void setTarget(InputConnection target) {
         if (mTarget != null && !mMutable) {
             throw new SecurityException("not mutable");
         }
         mTarget = target;
     }
-
+    
     public CharSequence getTextBeforeCursor(int n, int flags) {
         return mTarget.getTextBeforeCursor(n, flags);
     }