hidden_api: Call back into libcore on hidden api detection

This change also removes some unnecessary RI specific logic for
building src-ex since it isn't required.

Bug: 73896556
Test: run-test --host 674-hiddenapi
Test: StrictModeTest

Co-Authored-By: Andreas Gampe <agampe@google.com>
Change-Id: Ib2b4dfad55c5d829630bfe2adb4a468124bea61c