Java flag support for statsd

Declare statsd_java and statsd_java_boot as statsd flag namespaces for
java features.

Bug: 180992603
Test: used adb shell cmd device_config put to insert properties and
manually verified that statsd java code in system server could read.

Change-Id: I9e1ad337800f503b2f0df1c5e5eec9d752a9cbd5