Trapping backspace at EditText level to prevent popping Docs.

When the EditText is empty, pressing backspace on soft/hardware keyboard
will call on the EditText's OnKeyListener. Since we don't handle
backspace, it will then bubble up to Activity-level, which will then pop
the document stack due to the feature of using backspace as navigation.
This will trap the call correctly if the EditText is empty.

Bug: 30066261
Change-Id: I231507b9469bfa52478872491f1d2713a1ac58ba

packages/DocumentsUI/src/com/android/documentsui/SaveFragment.java

@@ -22,6 +22,7 @@ import android.app.FragmentTransaction;
 import android.content.Context;
 import android.os.Bundle;
 import android.text.Editable;
+import android.text.TextUtils;
 import android.text.TextWatcher;
 import android.view.KeyEvent;
 import android.view.LayoutInflater;
@@ -91,6 +92,14 @@ public class SaveFragment extends Fragment {
                             return false;
                         }
 
+                        // Returning false in this method will bubble the event up to
+                        // {@link BaseActivity#onKeyDown}. In order to prevent backspace popping
+                        // documents once the textView is empty, we are going to trap it here.
+                        if (keyCode == KeyEvent.KEYCODE_DEL
+                                && TextUtils.isEmpty(mDisplayName.getText())) {
+                            return true;
+                        }
+
                         if (keyCode == KeyEvent.KEYCODE_ENTER && mSave.isEnabled()) {
                             performSave();
                             return true;