Fix hidden API flags in com.android.i18n

Change 70cfdff3 changed the hidden API
flags in com.android.i18n as it stopped the i18n-bootclasspath-fragment
from making the hidden API flag files available for use by
platform-bootclasspath.

This change fixes that by exporting the flag files even if hidden API
flag generation is skipped.

Bug: 179354495
Test: m com.android.i18 out/soong/hiddenapi/hiddenapi-flags.csv
      - make sure that the flags in
        packages/modules/RuntimeI18n/apex/hiddenapi/hiddenapi-max-target-o-low-priority.txt
        are reflected in the core-icu4j dex files in the apex.
Change-Id: I9b5c7c74bd996ab447bc0e0452da5fd49191a35d