Expand TestableDeviceConfigTest#getProperties().

Fixes: 151023102
Test: atest services/tests/mockingservicestests/src/com/android/server/testables/TestableDeviceConfigTest.java
Change-Id: I2d8acd4947ac687cc5e91756708b53ae05b5962e
diff --git a/services/tests/mockingservicestests/src/com/android/server/testables/TestableDeviceConfigTest.java b/services/tests/mockingservicestests/src/com/android/server/testables/TestableDeviceConfigTest.java
index ba995e4..0e40669 100644
--- a/services/tests/mockingservicestests/src/com/android/server/testables/TestableDeviceConfigTest.java
+++ b/services/tests/mockingservicestests/src/com/android/server/testables/TestableDeviceConfigTest.java
@@ -122,6 +122,12 @@
         properties = DeviceConfig.getProperties(sNamespace, sKey, newKey);
         assertThat(properties.getString(sKey, null)).isEqualTo(sValue);
         assertThat(properties.getString(newKey, null)).isEqualTo(newValue);
+
+        String unsetKey = "key3";
+        properties = DeviceConfig.getProperties(sNamespace, newKey, unsetKey);
+        assertThat(properties.getKeyset()).containsExactly(newKey, unsetKey);
+        assertThat(properties.getString(newKey, null)).isEqualTo(newValue);
+        assertThat(properties.getString(unsetKey, null)).isNull();
     }
 
     @Test