Update language to comply with Android's inclusive language guidance

See https://source.android.com/setup/contribute/respectful-code for reference

Bug: 161896447
Test: robotest & manual
Change-Id: I36829e0055437dab79d9e5339ac002a87a4b874f

src/com/android/settings/backup/SettingsBackupHelper.java

@@ -59,7 +59,7 @@ public class SettingsBackupHelper extends BackupAgentHelper {
 
             try (FileOutputStream out = new FileOutputStream(newState.getFileDescriptor())) {
                 if (getVersionCode(oldState) != VERSION_CODE) {
-                    data.writeEntityHeader("dummy", 1);
+                    data.writeEntityHeader("placeholder", 1);
                     data.writeEntityData(new byte[1], 1);
                 }