Fix a flaky test.

See b/152840581#comment2 for the analysis.

call #waitForDeviceNotAvailable to wait for reboot reliably.

Bug: 152840581
Test: atest NetworkStagedRollbackTest
Change-Id: Ibbc9998c74681b1b8fc29b90ea66406f39ae7443
diff --git a/tests/RollbackTest/Android.bp b/tests/RollbackTest/Android.bp
index 24623fb..2be4ae6 100644
--- a/tests/RollbackTest/Android.bp
+++ b/tests/RollbackTest/Android.bp
@@ -39,7 +39,7 @@
     name: "NetworkStagedRollbackTest",
     srcs: ["NetworkStagedRollbackTest/src/**/*.java"],
     libs: ["tradefed"],
-    static_libs: ["testng", "RollbackTestLib"],
+    static_libs: ["RollbackTestLib"],
     test_suites: ["general-tests"],
     test_config: "NetworkStagedRollbackTest.xml",
 }