mobile: AndroidEngineExplicitFlowTest is flaky

[fredyw]

To reproduce:

bazel test --runs_per_test=100 //test/java/integration:android_engine_explicit_flow_test

The error:

1) post_simple(test.java.integration.AndroidEngineExplicitFlowTest)
java.lang.AssertionError: Error code: 0, error message: 
 	at org.junit.Assert.fail(Assert.java:89)
 	at test.java.integration.AndroidEngineExplicitFlowTest.post_simple(AndroidEngineExplicitFlowTest.java:365)

What's strange is the error code is set to 0 and there's no error message.