SI-7603 Fix thread safety of FlagTranslation 

This is outside of the Global cake, so we can't assume single
threaded access. A var was introduced in af3daf6 that can lead
to incorrect flag interpretation. The reported bug was triggered
in a multi-project SBT build. Java Annotations read from classfiles
were occasionally conferred the TRAIT|INFERFACE|ABSTRACT flag set,
leading to `@Foo is abstract, cannot be instatiated` later down the
line.

Review by @JamesIry. I could add a test to prod those functions for thread-safety, but I don't think an isolated test like that adds much value to our test suite.

(This one will fix Typesafe tickets 2209 and 2323. I just got access to a build today that allowed the diagnosis, hence the eleventh hour submission for 2.10.3)