[RESTRICT AUTOMERGE] Add hide-non-system-overlay flag for HarmfulAppWarningActivity am: 2c87a8a7ce

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/16743363

Change-Id: Ifdf47e595d507cd8d67bb968b1234a5d11d2b387