Consolidate Java version checking code, fix bugs

...erconsole/java/src/net/i2p/router/update/NewsFetcher.java

...le/java/src/net/i2p/router/update/PluginUpdateRunner.java

...terconsole/java/src/net/i2p/router/web/PluginStarter.java

...sole/java/src/net/i2p/router/web/RouterConsoleRunner.java

core/java/src/net/i2p/util/SystemVersion.java