Prevent adding duplicate overlays

- Hypothetical fix: check for duplicate overlay before adding it to the
  overlay list, otherwise we may attempt to release the same overlay
  twice when cleaning up the overlay later. We keep the rest of the
  setup just to keep it consistent with adding a new overlay though.
- Hold the wm lock when checking the overlay surface package state
  as in ag/25193528 to ensure it's also valid for the duration of the
  WM code running

Bug: 314880864
Test: atest TrustedOverlayTests
Change-Id: Ib7cad1a2265ba82dd1d98b8358380874f9f8719d
Signed-off-by: Winson Chung <winsonc@google.com>