libtextclassifier-java should just use the core_current sdk

Some branches are broken by this error:
error: frameworks/base/Android.bp:809:1: module "ext" variant "android_common": compiles against core Java API, but dependency "libtextclassifier-java" is compiling against non-core Java APIs.Adjust sdk_version: property of the source or target module so that target module is built with the same or smaller API set than the source.

Reference: external/rappor/Android.bp

Somehow master branch can be built without this line, and treehugger can't spot this either.
So not sure what's the best way to test out the fix.

Test: None
Change-Id: I44e3afcb978fd6732375b72c8c998d264acd601f