Fix signature for finalize() to resolve API Lint error

The API linter on goog/master noticed that this method is
listed in the public API even though it's protected. The
change is probably related to a signature change from the
internal finalize method which throws a Throwable. Fix the
method in IpSecManager to throw Throwable, which should
fix the current.txt and resolve the lint error.

Bug: 69006767
Test: compilation, make update-api
Change-Id: I173d014baaa505c365b7916fcb52f2a8b4af9373