Add ALLOW_SLIPPERY_TOUCHES permission

This permission allows the app to use FLAG_SLIPPERY. This means, windows
of the app that has this permission can let touches slip out when the
finger moves out of the window bounds.

Bug: 157929241
Bug: 206188649
Test: atest FlagSlipperyTest
Change-Id: I9ccdfd298f32c36b9c4da68c2e9c355c97dc7593