Prevent hiddenapi entries being added for libcore related projects

The libcore related projects (see below) have been (mostly) switched
over to use UnsupportedAppUsage annotations, This change will prevent
entries for those projects being added to a config/hiddenapi-* file.

* libcore
* external/bouncycastle
* external/conscrypt
* external/icu
* external/okhttp
* external/libphonenumber - still has a couple of entries in
      config/hiddenapi-light-greylist.txt due to limitations in
      UnsupportedAppUsage/class2greylist.

Tested by attempting to upload the file with entries for libcore
projects and without those entries and checking that the behavior
is expected.

Test: see above
Bug: 117818301
Change-Id: I67a30b307e12e842b28cfb2160fab0029868fa06