Allow using files from local storage (#2200)

Summary:
Pull Request resolved: #2200

This helps us decouple models from the app, so we can use language model.
```
adb push language_llama_xnnpack_kv.pte /data/local/tmp/language.pte
adb push flores200sacrebleuspm.bin /data/local/tmp/language.bin
```

Reviewed By: mcr229

Differential Revision: D54427423

fbshipit-source-id: 35ebe58c04c8f817750cee72bdc51834db0d2866

Author: kirklandsign

examples/demo-apps/android/LlamaDemo/app/src/main/java/com/example/executorchdemo/MainActivity.java

@@ -68,6 +68,10 @@ private void setModel(String modelPath, String tokenizerPath) {
     }
   }
 
+  private void setLocalModel(String modelPath, String tokenizerPath) {
+    mModule = new LlamaModule(modelPath, tokenizerPath, 0.8f);
+  }
+
   private void modelDialog() {
     AlertDialog.Builder builder = new AlertDialog.Builder(this);
     builder.setTitle("Select a Model");
@@ -81,7 +85,7 @@ public void onClick(android.content.DialogInterface dialog, int item) {
                 setModel("stories110M.pte", "tokenizer.bin");
                 break;
               case 1:
-                setModel("language.pte", "language.bin");
+                setLocalModel("/data/local/tmp/language.pte", "/data/local/tmp/language.bin");
                 break;
             }
             mEditTextMessage.setText("");