TtsService.java: use the context from the TtsService object itself

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

 
     @Override
     protected int onIsLanguageAvailable(String language, String country, String variant) {
-        Context context = getApplicationContext();
         if (!mEngineInitialized ||
-            !CheckVoiceData.hasBaseResources(context) ||
-            CheckVoiceData.canUpgradeResources(context)) {
+            !CheckVoiceData.hasBaseResources(this) ||
+            CheckVoiceData.canUpgradeResources(this)) {
             return TextToSpeech.LANG_MISSING_DATA;
         }