tests: Update string for measuring pvmfw boot time

Replace the U-Boot boot message by the one we have in Rust pvmfw, to
properly measure the time spent on pvmfw during boot.

Bug: 237371962
Test: atest ComposBenchmarkApp
Change-Id: I0fcea98c7828b6e4fe313af1f9a09c0b8e9703c7
diff --git a/tests/helper/src/java/com/android/microdroid/test/device/MicrodroidDeviceTestBase.java b/tests/helper/src/java/com/android/microdroid/test/device/MicrodroidDeviceTestBase.java
index 2deeb70..c2fd295 100644
--- a/tests/helper/src/java/com/android/microdroid/test/device/MicrodroidDeviceTestBase.java
+++ b/tests/helper/src/java/com/android/microdroid/test/device/MicrodroidDeviceTestBase.java
@@ -150,7 +150,7 @@
             if (!mVcpuStartedNanoTime.isPresent()) {
                 mVcpuStartedNanoTime = OptionalLong.of(System.nanoTime());
             }
-            if (log.contains("Starting kernel") && !mKernelStartedNanoTime.isPresent()) {
+            if (log.contains("Starting payload...") && !mKernelStartedNanoTime.isPresent()) {
                 mKernelStartedNanoTime = OptionalLong.of(System.nanoTime());
             }
             if (log.contains("Run /init as init process") && !mInitStartedNanoTime.isPresent()) {