Updater: Show toast when starting export

* It's not immediately clear that an export is started since the
  notification isn't intrusive
* Show a toast - that will be clear enough

Change-Id: I78d78b23884df3ddb37bc486c1d6012f899c888d

res/values/strings.xml

@@ -125,6 +125,7 @@
     <string name="notification_export_success">Update exported</string>
     <string name="notification_export_fail">Export error</string>
     <string name="toast_already_exporting">Already exporting an update</string>
+    <string name="toast_export_started">Export started</string>
 
     <plurals name="eta_seconds">
         <item quantity="one">1 second left</item>

src/org/lineageos/updater/ExportUpdateService.java

@@ -69,6 +69,7 @@ public class ExportUpdateService extends Service {
             File source = (File) intent.getSerializableExtra(EXTRA_SOURCE_FILE);
             Uri destination = intent.getParcelableExtra(EXTRA_DEST_URI);
             startExporting(source, destination);
+            Toast.makeText(this, R.string.toast_export_started, Toast.LENGTH_SHORT).show();
         } else {
             Log.e(TAG, "No action specified");
         }