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_unittest.cc b/update_attempter_android_unittest.cc
index 2593d44..3be0b7e 100644
--- a/update_attempter_android_unittest.cc
+++ b/update_attempter_android_unittest.cc
@@ -111,9 +111,10 @@
   update_attempter_android_.Init();
   // Check that we reset the metric prefs.
   EXPECT_FALSE(prefs_.Exists(kPrefsNumReboots));
-  EXPECT_FALSE(prefs_.Exists(kPrefsPayloadAttemptNumber));
   EXPECT_FALSE(prefs_.Exists(kPrefsUpdateTimestampStart));
   EXPECT_FALSE(prefs_.Exists(kPrefsSystemUpdatedMarker));
+  // PayloadAttemptNumber should persist across reboots.
+  EXPECT_TRUE(prefs_.Exists(kPrefsPayloadAttemptNumber));
 }
 
 TEST_F(UpdateAttempterAndroidTest, ReportMetricsOnUpdateTerminated) {