Revert policy looping logic back to iterating over copy of PolicyKeys instead...

Revert policy looping logic back to iterating over copy of PolicyKeys instead of directly on the values.

Changes made in I3daf12ee9eaebd28b187a6ff03b7134127010f34 accidentally
resulted in a ConcurrentModificationException.

Test: atest SupervisionServiceTest
Test: manual
Bug: 434981305
Flag: android.app.supervision.flags.supervision_manager_apis
Change-Id: I5999a3b96e706f5a67fcce7516d23617bf6e1921