Only allow trusted overlays to specify FLAG_SLIPPERY am: 07e7aaff29

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/16190719

Change-Id: If208e3c8b5a9d43034b4764d678a1a33ea2729c6
Merged-In: Ia30f1c38d5ddb351c90b748ea76448a76a9dde7b
Bug: 157929241
Bug: 205996845