Don't allow a zero-length device name to work.

Technically, if a device has a zero-length device name, the bug will
resurface. The EditText validator doesn't trigger on opening -- only
when the text is edited. A zero-length device name is flagged as being
invalid, but that fails if the text box starts empty.

By pre-filling it with the previous device name, we can ensure that, as
long as a zero-length device name is never set, it can never be set.

Change-Id: I0d28aaae09f99b7d697b753835ba39c0c06644a1
Fixes: 73127912
Test: Robotest
(cherry picked from commit b216550944)

src/com/android/settings/deviceinfo/DeviceNamePreferenceController.java

@@ -54,7 +54,9 @@ public class DeviceNamePreferenceController extends BasePreferenceController
     public void displayPreference(PreferenceScreen screen) {
         super.displayPreference(screen);
         mPreference = (ValidatedEditTextPreference) screen.findPreference(PREF_KEY);
-        mPreference.setSummary(getSummary());
+        final CharSequence deviceName = getSummary();
+        mPreference.setSummary(deviceName);
+        mPreference.setText(deviceName.toString());
         mPreference.setValidator(this);
     }
 

tests/robotests/src/com/android/settings/deviceinfo/DeviceNamePreferenceControllerTest.java

@@ -123,4 +123,12 @@ public class DeviceNamePreferenceControllerTest {
         verify(mWifiManager).setWifiApConfiguration(captor.capture());
         assertThat(captor.getValue().SSID).isEqualTo(TESTING_STRING);
     }
+
+    @Test
+    public void displayPreference_defaultDeviceNameIsModelNameOnPreference() {
+        mController.displayPreference(mScreen);
+
+        assertThat(mPreference.getText()).isEqualTo(Build.MODEL);
+    }
+
 }