Delete android.net.PskKeyManager

android.net.PskKeyManager was @remove'd in Android O (API Level 26)
which means it may still be available on Android O platform, but is
not part of its public API. This commit deletes this class from
future platform versions.

The reasoning for the removal of this class is that:
1. it is incompatible with TLS 1.3, and
2. TLS-PSK cannot be exposed as a standalone primitive in a safe by
   default way. There is no way for such primitive to ensure that
   it is used with a shared secret which is not hard-coded into the
   app or is otherwise publicly known.

Test: make
Test: Run a test app which uses PskKeyManager -- the app crashes
      because the class is not found
Bug: 34722996
Change-Id: I5f9a1a1784004b1387ef756eadf8fa06796c96bc