Add missing compatibility check for older android versions. (#1472)

Note: this doesn't fix actual problem for some of the older devices.
Unfortunately i don't have devices with os version below than android 7,
so am not able to test.

android/src/com/reecedunn/espeak/TtsService.java

     @Override
     public void onCreate() {
         storageContext = EspeakApp.getStorageContext();
-        storageContext.moveSharedPreferencesFrom(this, this.getPackageName() + "_preferences");
+        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N)
+            storageContext.moveSharedPreferencesFrom(this, this.getPackageName() + "_preferences");
         initializeTtsEngine();
         super.onCreate();
     }