Add PrepareUpdateService.

It's moved from PrepareStreamingService intent service.
Now PrepareUpdateService takes an UpdateConfig and
builds PayloadSpec for UpdateEngine for both streaming
and non-streaming update.

It allows us to do all preparations in intent service's
thread, without blocking UI.

We will also add checksum verification to
PrepareUpdateService.

Test: device, junit
Bug: 77150191
Change-Id: Iea69acd9aa41e17538c26aff60f7598093ca7744
diff --git a/updater_sample/src/com/example/android/systemupdatersample/util/FileDownloader.java b/updater_sample/src/com/example/android/systemupdatersample/util/FileDownloader.java
index ddd0919..0f9083d 100644
--- a/updater_sample/src/com/example/android/systemupdatersample/util/FileDownloader.java
+++ b/updater_sample/src/com/example/android/systemupdatersample/util/FileDownloader.java
@@ -30,7 +30,7 @@
  * Downloads chunk of a file from given url using {@code offset} and {@code size},
  * and saves to a given location.
  *
- * In real-life application this helper class should download from HTTP Server,
+ * In a real-life application this helper class should download from HTTP Server,
  * but in this sample app it will only download from a local file.
  */
 public final class FileDownloader {