Fix ErrorProne related build error

Error:  Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.13.0:compile (default-compile) on project exec: Fatal error compiling: The default --should-stop=ifError policy (INIT) is not supported by Error Prone, pass --should-stop=ifError=FLOW instead -> [Help 1]

<arg>--should-stop=ifError=FLOW</arg>

does the trick, as per
https://errorprone.info/docs/installation#maven

see google/error-prone#4705

pom.xml

@@ -65,6 +65,7 @@
             <arg>-Werror</arg>
             <arg>-Xlint:all,-options</arg>
             <arg>-XDcompilePolicy=simple</arg>
+            <arg>--should-stop=ifError=FLOW</arg>
             <arg>-Xplugin:ErrorProne -XepAllDisabledChecksAsWarnings
               -Xep:BooleanParameter:OFF -Xep:AndroidJdkLibsChecker:OFF
               -Xep:Java7ApiChecker:OFF -Xep:Varifier:OFF</arg>