clang-format: [Java] Never treat @interface as annotation.

'@' followed by any keyword can't be an annotation, but @interface is currently
the only combination of '@' and a keyword that's allowed, so limit it to this
case. `@interface Foo` without a leading `public` was misformatted prior to
this patch.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@221607 91177308-0d34-0410-b5e6-96231b3b80d8