Add OWNERS file for frameworks/base/tools/localedata/extract_icu_data.py

This folder has only one script extract_icu_data.py.
The script parses the ICU data and generates a .cpp file.
Thus, add external/icu/OWNERS as the owner here.

Test: treehugger
Change-Id: I555e9785c576688e43e6362050180345c81d6c3e