Metrics: PayloadAttemptNumber should persist across attempts

We should only reset the PayloadAttemptNumber if the update succeeds, or
we switch to a different payload. The old code has already checked for
the payload id; and we should only delete this pref upon successful
updates.

Bug: 122841006
Test: unit tests pass
Change-Id: I7743b721d2899ba59375f8465468cc0e2a9b3568
diff --git a/update_attempter_android.h b/update_attempter_android.h
index e4b40de..c4710ad 100644
--- a/update_attempter_android.h
+++ b/update_attempter_android.h
@@ -151,9 +151,9 @@
   void UpdatePrefsOnUpdateStart(bool is_resume);
 
   // Prefs to delete:
-  //   |kPrefsNumReboots|, |kPrefsPayloadAttemptNumber|,
+  //   |kPrefsNumReboots|, |kPrefsCurrentBytesDownloaded|
   //   |kPrefsSystemUpdatedMarker|, |kPrefsUpdateTimestampStart|,
-  //   |kPrefsUpdateBootTimestampStart|, |kPrefsCurrentBytesDownloaded|
+  //   |kPrefsUpdateBootTimestampStart|
   void ClearMetricsPrefs();
 
   DaemonStateInterface* daemon_state_;