Add NPE check for method assignDefaultPhoto in Utils.java

When user press back button while adding user, app may crash because
of null context. In this cl, I add NPE check for context in method
assignDefaultPhoto and change this method to return boolean value.
If the return value is false, then stop the following steps(e.g.
creating a dialog after pressing button may also cause crash.)

Bug: 33829308
Test: make -j40 RunSettingsRoboTests
Change-Id: I811b92c268fb20f8c43ab3b0a9ceebe76f5d83be

tests/robotests/src/com/android/settings/UtilsTest.java

@@ -65,4 +65,10 @@ public class UtilsTest {
 
         assertThat(Utils.getWifiIpAddresses(mContext)).isNull();
     }
+
+    @Test
+    public void testAssignDefaultPhoto_ContextNull_ReturnFalseAndNotCrash() {
+        // Should not crash here
+        assertThat(Utils.assignDefaultPhoto(null, 0)).isFalse();
+    }
 }