Use the build start time from Soong main UI for metrics build timestamp.

Currently, the build timestamp saved to the build_date_timestamp
field in the MetricsBase metrics protobuf usually comes from a text
file named build_date.txt. It may not be accurate for metrics purposes.
Instead, use the build start time from Soong main UI for a better
timestamp.

Bug: b/140638454
Test: Ran and checked the build_date_timestamp by running the printproto
      command.
Change-Id: I4d893fbbf9830ba21911e56ae13bc5272be47ae6
Merged-In: I4d893fbbf9830ba21911e56ae13bc5272be47ae6
diff --git a/ui/build/upload_test.go b/ui/build/upload_test.go
index c812730..dccf156 100644
--- a/ui/build/upload_test.go
+++ b/ui/build/upload_test.go
@@ -97,7 +97,7 @@
 				buildDateTime: strconv.FormatInt(time.Now().UnixNano()/int64(time.Millisecond), 10),
 			}}
 
-			UploadMetrics(ctx, config, false, 1591031903, metricsFiles...)
+			UploadMetrics(ctx, config, false, time.Now(), metricsFiles...)
 		})
 	}
 }
@@ -151,7 +151,7 @@
 					"OUT_DIR=/bad",
 				}}}
 
-			UploadMetrics(ctx, config, true, 1591031903, metricsFile)
+			UploadMetrics(ctx, config, true, time.Now(), metricsFile)
 			t.Errorf("got nil, expecting %q as a failure", tt.expectedErr)
 		})
 	}